Ignore effectless indexable/unindexable state transitions

During rebalance ns_server keeps asking to mark a group of partitions as indexable when they're all already indexable (or when they're all unindexable already).
This causes updater, compactor and cleaner processes to be restarted, and an header to be built and written to the index file, as the view engine wasn't accounting for such useless calls.

Such effectless and useless calls, should just be a no-op with no side effects.