Add a new config `drop_backend_after_decommission` in FE. if this config
is false, the BE will not be dropped after finishing decommission operation.
This new config is try to solve the problem described in ISSUE: #3460 .
TODO:
This method will generate a lot of data migration, so it is only a temporary solution.
After that, we should try to solve the problem of data balancing within the BE.
This CL also add the documents of FE and BE configuration.
These documents are incomplete and can be added later.
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