pool: Add another level of indirection to the pool internal logic

Make internal connection pool queue reference a new intermediate
object -- PoolConnectionHolder -- instead of Connections directly.

PoolConnectionHolder fully encapsulates the management of one
connection:

* connection logic;
* handling closed connections;
* handling connections that exceeded max-queries quota; etc.

The new logic guarantees that old and new connections are always
added back to the pool (1), removes a deadlock in pool.acquire (2),
uses a LIFO queue to take advantage of "min_size" pool
parameter (3).

Author: 1st1