Improve PostgreSQL implementation of Shard API

[guilload]

• [x] simplify proto messages that don't require multi-source support (AcquireShardsRequest, TruncateShardsRequest)
• [x] implement sqlx::Encode for IndexUid, Position, ShardId, ShardState
• [ ] batch queries instead of using for-loops whenever possible
• [ ] report partial failures for non-transactional requests
• [x] returning error instead of logging warning if shard is not deletable
• [ ] return error if index or source does not exist
• [ ] re-enable gRPC metastore test suite