Support multi-disks for PageStorage

[flowbehappy]

Currently, we can only use one path to store a PageStorage. It causes much more IO pressure(from Delta and Region snapshots) for the first disk than other disks when TiFlash is deployed in multi-paths mode. And support multi-paths for PageStorage can distribute the IO pressures between all disks.

Tasks:

• Support multi-disks for DeltaTree's delta data
• Support multi-disks for Region data (in review)
• Use the global available size for choosing new path for storing DeltaTree's delta/stable and Region data
• Refactor related configurations
• TiUP support the new configurations Support deploy TiFlash on multi-disks with "storage" configurations since v4.0.9 tiup#931
• Documents
  • Chinese: Add desc of new TiFlash storage settings and guidelines for upgrading in v4.0.9 docs-cn#4965
  • English: tiflash: Add desc of new TiFlash storage settings and guidelines for upgrading in v4.0.9 docs#4290

[JaySon-Huang]

Design doc：https://docs.google.com/document/d/1281wqqWNVj6LZMiWfcBPPQLOQN7w3koODPVLYWM6E68/edit#

[JaySon-Huang]

The IOPS in StoragePool.meta_storage is almost the same as StoragePool.log_storage, though meta_storage requires less bandwidth.
To get better performance under multi-disks deployment, we should distribute the IOPS pressure from meta_storage.

[JaySon-Huang]

Now StoragePathPool only considers the used size when choosing disks for new DTFile/PageFile. It may make the data imbalance between different disks.
We may use the used/capacity in PathCapacityMetrics instead of the used size in each Storage.

[JaySon-Huang]

## Deprecated storage path setting style. Check [storage] section for new style.
# path = "/tmp/tiflash/data/db"
# capacity = "10737418240"
## Deprecated storage path setting style of multi-disks. Check [storage] section for new style.
# path = "/tmp/tiflash/tiflash0,/tmp/tiflash1,/tmp/tiflash2"
## If you set `path_realtime_mode` to `true` and multiple directories are deployed in
## the path, the latest data is stored in the first directory and older data is stored in
## the rest directories.
# path_realtime_mode = false
# capacity = "0,0,0"

# new style storage paths
[storage]
## If there are multiple SSD disks on the machine,
## specify the path list on `storage.main.dir` can improve TiFlash performance.

## If there are multiple disks with different IO metrics (e.g. one SSD and some HDDs)
## on the machine,
## set `storage.latest.dir` to store the latest data on SSD (disks with higher IOPS metrics)
## set `storage.main.dir` to store the main data on HDD (disks with lower IOPS metrics)
## can improve TiFlash performance.

[storage.main]
## The path to store main data.
# e.g.
# dir = [ "/data0/tiflash" ]
# or
# dir = [ "/data0/tiflash", "/data1/tiflash" ]

## Store capacity of each path, i.e. max data size allowed.
## If it is not set, or is set to 0s, disk capacity will be used.
# capacity = [ ]

[storage.latest]
## The path(s) to store latest data.
## If not set, it will be the same with `storage.main.dir`.
dir = [ ]

## Store capacity of each path, i.e. max data size allowed.
## If it is not set, or is set to 0s, disk capacity will be used.
# capacity = [ ]

[storage.raft]
## The path(s) to store Raft data.
## If not set, it will be the paths in `storage.latest.dir` appended with "/kvstore".
# dir = [ ]

I think we'd better use the list format in TOML instead of a string separated with ",".
And the capacity should be a list for paths too.
If the disks are 500GB nvme SSD * 1, 2 TB HDD * 10, we need to set different capacities for each path.

What's more, we should choose a path for DTFile/PageFile by the global free disk space instead of the used disk space of each storage.

[JaySon-Huang]

storage:
    ## If there are multiple SSD disks on the machine,
    ## specify the path list on `storage.main.dir` can improve TiFlash performance.

    ## If there are multiple disks with different IO metrics (e.g. one SSD and some HDDs)
    ## on the machine,
    ## set `storage.latest.dir` to store the latest data on SSD (disks with higher IOPS metrics)
    ## set `storage.main.dir` to store the main data on HDD (disks with lower IOPS metrics)
    ## can improve TiFlash performance.

    main:
        ## The path to store main data.
        dir: 
        - /data1/tiflash
        - /data2/tiflash
        ## Store capacity of each path, i.e. max data size allowed.
        ## If it is not set, or is set to 0s, disk capacity will be used.
        capacity:
        - 107374182400
        - 107374182400
    latest:
        ## The path(s) to store latest data.
        ## If not set, it will be the same with `storage.main.dir`.
        #dir:
        #    - ""
        ## Store capacity of each path, i.e. max data size allowed.
        ## If it is not set, or is set to 0s, disk capacity will be used.
        #capacity:
        #    - 0
    raft:
        ## The path(s) to store Raft data.
        ## If not set, it will be the paths in `storage.latest.dir` appended with "/kvstore".
        #dir:
        #    - ""

The configuration in topo.yaml file of TiUP should be in this format.