Add LIFO for connection pooling

Add LIFO for connection pooling

Added new "lifo" mode to :class:`.QueuePool`, typically enabled by setting
the flag :paramref:`.create_engine.pool_use_lifo` to True.   "lifo" mode
means the same connection just checked in will be the first to be checked
out again, allowing excess connections to be cleaned up from the server
side during periods of the pool being only partially utilized.  Pull request
courtesy Taem Park.

Change-Id: Idb5e299c5082b3e6b547bd03022acf65fdc34f35
Pull-request: https://github.com/zzzeek/sqlalchemy/pull/467