Following WT-5403, all the operations on a transaction obtain the transaction access through a cursor.

This ticket will add a LAS transaction to the session and change the LAS API and implementation to use that transaction. We will do away with having separate LAS sessions and each session with get a LAS transaction and a LAS cursor of its own.

For now we can open the cursor on LAS when first needed or when the session gets created and close when the session gets destroyed. Do whichever is easier to begin with.
We will double the transaction states in txn_global, two for each session now. All the code that walks all the transactions will be changed to walk double the number of states.