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!
Added a test to verify sort ordering for binary encoded MVCC metadata
keys and versioned value keys.
Merge pull request #57 from cockroachdb/spencerkimball/range-key-tests

Added a test to verify sort ordering for binary encoded MVCC metadata
Add another test case.
Merge pull request #58 from cockroachdb/spencerkimball/range-key-tests

Add another test case.
Merge pull request #56 from cockroachdb/spencerkimball/delete-range-to-read-timestamp-cache

Always update timestamp cache. This correctly resolves our final commit
compute split key for range splitting

moved splitting into mvcc