Add SET constraintCheckTime = 'X' which forwards to the TransactionService, which ignores it for Persistit.
Clean up handling of allowed SET options, which was getting kinda large and still had newtypes in it.
merge MMcM: Add SET constraintCheckTime

Merge pull request #1033 from MMcM/sql-layer/xact-set-session-option
https://github.com/FoundationDB/sql-layer/pull/1033