Remove fake _get_part and use the real thing

The get_part method is fast and stable given a consistent hash path
suffix/prefix, so there's no absolute requirement for the fake
implementation other than convenience.  OTOH, removing the fake
implementation and fixing the tests that were relying on it should make
it easier to write better tests going forward and harder to hide bugs
that don't show up when using the fakes.

There may be some overhead when writing new tests that use the ring if
you're making assertions on partitions or paths, but with a part power
of zero it's normally trivially obvious when a 1 needs to be a 0 or vice
versa.  Or you can just drop the assertions around the parts you were
faking anyway.

Change-Id: I8bfc388a04eff6491038991cdfd7686c9d961545
Merge "Remove fake _get_part and use the real thing"