Citus Helper Function

Citus didn't come with some necessary function by default. I wrote some function to help maintenance jobs easier.

1. get_inactive_shard(): Query to show shard that become inactive.
2. repair_shard(): Query to repair ALL shard by replace the healthy shard to inactive shard.
3. repair_single_shard(shard_id): Query to repair single shard (expect shard_id as a parameter).