spanner: Session pool should hand out sessions in LIFO order

[skuruppu]

Is your feature request related to a problem? Please describe.
Currently sessions are handed out from the session pool in FIFO order. Least recently used sessions are unlikely to be in backend caches, and the transactions associated with them are likely to aborted due to idleness.

Describe the solution you'd like
The session pool should hand out sessions in LIFO order.

[tseaver]

For the FixedSizePoot and BurstyPool classes, this change is straightforward: we just use queue.LifoQueue instead of queue.Queue for the sessions.

For the PingingPool and TransacctionPingingPool class, the issue isn't quite clear: sessions are held in a priority queue based on when they need to be "pinged" (checked for existence at intervals).