Switch to in-memory sqlite for unit tests; 1500% speed improvement.

Switch to in-memory sqlite for unit tests; 1500% speed improvement.

All unit tests now run in 4 seconds, instead of 64 seconds on a HDD.

 _ENGINE global setting had to be moved into get_engine() so that migration works, otherwise get_engine() creates a new isolated in-memory database for
every call.

Other unit test speedups in this change are to stub out some eventlet sleeps
which saves about 5 seconds.

Change-Id: I3643b73dd9bd86c414934b7c78db67402587f570