Currently, new custom shards are initialized directly in Active state. In this case peer A (that did not receive custom shard replica) might expect replica to already exist on peer B, but peer B might still be initializing local replica of new custom shard. This leads to annoying errors during search while custom shard is being created.

This PR tries to mitigate this by initializing replicas in Inititalizing state (so that each peer would not assume that remote replicas are instantly available) and then switching each replica into Active state after it's initialized.

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