Use upstream patched Pool.get

This works around an eventlet bug in eventlet 0.9.16.
This version properly keeps track of pool size accounting, and
therefore doesn't let the pool grow without bound. This patched
version is the result of commit
f5e5b2bda7b442f0262ee1084deefcc5a1cc0694 in eventlet and is
documented at https://bitbucket.org/eventlet/eventlet/issue/91

This patch includes full test coverage of the back-ported code, even
when the actually-installed eventlet is newer.

This fixes bug #1254119

Change-Id: I075bb5e40e08571d52fe17fcc3fa0e25be5befed
Merge "Use upstream patched Pool.get"