[Resharding] Omnisharded tables sync

[levkk]

Add the ability to synchronize omnisharded tables between shards of:

1. the same cluster
2. two different clusters

This should re-use all of our existing code for schema sync, data sync and replication, but do so only for omnisharded tables.

API

1. Add --omnisharded-only flag to schema-sync and data-sync commands.
2. Add omnisharded_only to COPY_DATA and SCHEMA_SYNC admin database commands

[rlittlefield]

I have some of this working in rlittlefield#2 (but only in the context of adding a new shard to an existing cluster, which is probably only safe on ranges and lookup_query mappings.