feat: add copy secondary for cluster balance

[levy5307]

Manual Test

1. start onebox with 5 replica servers and 3 meta servers

 ./run.sh start_onebox -r 5 -m 3

2. create several tables with 4 partitions, for example:

>>> create test1
create app test1 succeed, waiting for app ready
test1 not ready yet, still waiting... (0/4)
test1 not ready yet, still waiting... (2/4)
test1 not ready yet, still waiting... (2/4)
test1 not ready yet, still waiting... (2/4)
test1 not ready yet, still waiting... (2/4)
test1 not ready yet, still waiting... (2/4)
test1 is ready now: (4/4)
test1 is ready now!
create app "test1" succeed

3. get information about load balance, and we can see the cluster is not balanced

>>> nodes -d
[details]
address              status    replica_count  primary_count  secondary_count
10.231.57.100:34801  ALIVE                28              9               19
10.231.57.100:34802  ALIVE                25              9               16
10.231.57.100:34803  ALIVE                28              8               20
10.231.57.100:34804  ALIVE                26              9               17
10.231.57.100:34805  ALIVE                25              9               16

4. enable cluster load balance

http://127.0.0.1:34601/updateConfig?balance_cluster=true

5. set meta_level to lively to enable load balance

>>> set_meta_level lively
control meta level ok, the old level is fl_steady

6. get information about load balance, and now the cluster is balanced.

>>> nodes -d
[details]
address              status    replica_count  primary_count  secondary_count
10.231.57.100:34801  ALIVE                27              9               18
10.231.57.100:34802  ALIVE                26              9               17
10.231.57.100:34803  ALIVE                26              8               18
10.231.57.100:34804  ALIVE                27              9               18
10.231.57.100:34805  ALIVE                26              9               17

[levy5307]

Here is the last pull request for cluster load balance :)