Always update timestamp cache. This correctly resolves our final commit
timestamp as we send writes to various nodes as part of a distributed
txn.

Renamed read_timestamp_cache to just timestamp_cache and updated comments
leading reader to believe it's just for reads!
Merge pull request #56 from cockroachdb/spencerkimball/delete-range-to-read-timestamp-cache

Always update timestamp cache. This correctly resolves our final commit