Make shard name explicit in the metadata

So far, we always generate the shard name by combining the table name with the shard ID. It would be useful if the shard name was explicitly stored in the metadata, such that it can be a generic string.

This has two immediate benefits:
- Federation: Use user-defined tables as shards
- Fixing our long names issue for new tables https://github.com/citusdata/citus/issues/1664

A challenge lies in naming indexes and updating some of the RPC like worker_apply_ddl_command.