Refactor meta part table

[critical27]

We only save who has given partition in meta for now, this works fine in normal cases. But during balance data, the part allocation will be modified once, for example, part 1 is on [A, B, C] and we want to replace A with D:

...
open part 1 on D
add D as learner
add D as peer
remove A from peer
modify meta from [ABC] to [BCD]

Actually, we need to handle the meta not only in last step, add learner/add peer/remove peer also need to handle it as well. What we need to do is that add a flag to indicate whether a host is learner or not.

Some places need to be modified, almost everywhere we use the part table in meta

1. create space: all part is not learner
2. balance data:
   • when we add learner, add D into the kv as learner
   • when we promote D as normal replica, modify D as normal one in kv
   • when we remove A from raft, modify the kv as well
3. start up process, since the meta table is modified, when starting up, we need to know my raft peers, some of then maybe learner, we need to use all data on meta.

[critical27]

After talked with @pengweisong offline, we could use a simple method to address this issue. Save the peer info in storage local. This won't involve too much changes in meta.