Move quadratic backoff and deadline handling in separate method.
Do not sleep past deadline.
Simplify _wait usage.

Preserve "blocking" parameter of "acquire" for standard python library
compatibility.
Add "timeout" parameters to "release".

Allows release to wait for memcached availability.
TestSwarm: opportunist test of uid unicity.
TestSwarm: close and join pool even when test fails.

Avoids propagating failure to following tests.
Add TestSlowSwarm.
Retry 3 times when network issue is suspected.

Once is not enough to make TestSwarm pass reliably (on my machine).