Schema cache

[erikgrinaker]

The KV SQL engine should cache schema lookups.

This needs to be implemented at the MVCC level, so that versioning works - probably by specifying prefixes that should be cached. The downside of this is that it needs to go through serialization.

[erikgrinaker]

Will drop this. The primary SQL engine is in-memory anyway, so a cache would only save the deserialization cost, which we may not be able to avoid anyway due to MVCC versioning.