- A major improvement made to the mechanics by which the :class:`.Engine`

- A major improvement made to the mechanics by which the :class:`.Engine`
recycles the connection pool when a "disconnect" condition is detected;
instead of discarding the pool and explicitly closing out connections,
the pool is retained and a "generational" timestamp is updated to
reflect the current time, thereby causing all existing connections
to be recycled when they are next checked out.   This greatly simplifies
the recycle process, removes the need for "waking up" connect attempts
waiting on the old pool and eliminates the race condition that many
immediately-discarded "pool" objects could be created during the
recycle operation. fixes #2985