Create condition based rebalancer

[frankmu]

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resolves #2822

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This PR initiates the condition based rebalancer which can perform restricted shard shuffling based on conditions. Will add more tests if the overall skeleton makes sense.

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[junkaixue]

Need the test add for this PR.

In addition, thinking about where to put the configuration for the rebalancer. My haunch is in IdealState simple fields.

[junkaixue]

I think we need a generic string -> string map for input ? Or directly pass the cache object.

Because condition is a piece of code logic but the real computation is happening at runtime with input.

[junkaixue]

lgtm

[frankmu]

This PR is ready to be merged. Approved by @junkaixue
Commit message: [Create condition based rebalancer]