Add the WT_EXTENSION_API.transaction_oldest method to return the oldest
transaction ID not yet visible to a running transaction.
Make unresolved transaction return state explicit, minimize read buffer size.
Try and make the error macro names a little more intuitive.
Add the cache resolution code for Memrata transactions.
Create a WT_CURSOR_DATA_SOURCE structure so data-source cursors (almost)
follow the standard cursor patterns.