Add support for timestamp in Sentinel support

[suhailpatel]

This PR adds a sentinel value for timestamp clustering columns. It chooses 1753-01-01 as the sentinel value. This is necessary because we can have clustering columns that are timestamps but certain stores (like Keyspaces) don't support the empty timestamp value as a clustering key

The decision to use 1st January 1753 (1753-01-01) as the minimum date value for a datetime in SQL Server goes back to its Sybase origins.

I've also made some methods public so we can use them elsewhere for other bits of internal code that check for sentinel values