Optimize the ring builder's _reassign_parts() method.

Another ring builder optimization.  Profiling revealed hotspots in many
calls to min() and list.sort() in _reassign_parts().  That method didn't
get exercised in my last optimization pass because that pass targeted a
rebalance where nothing really moved around.

This time, I wrote a script which created a fresh ring, added a bunch of
devices, did the initial balance, deleted some devices, balanced, and
added some more back in.

Results from homebrew Python 2.7.3 on OS X 10.8.2 Macbook Pro
(bare-metal):

 BEFORE:
 Using part-power = 18, adding 600 devices, removing 100, then adding 300 more...
 NOT Profiling to 'initial_balance.prof'
   wall-time delta: 131.33s
 NOT Profiling to 'deleting_200_rebalance.prof'
   wall-time delta: 25.67s
 NOT Profiling to 'first_rebalance.prof'
   wall-time delta: 62.00s

 AFTER:
 Using part-power = 18, adding 600 devices, removing 100, then adding 300 more...
 NOT Profiling to 'initial_balance.prof'
   wall-time delta: 28.04s
 NOT Profiling to 'deleting_200_rebalance.prof'
   wall-time delta: 9.35s
 NOT Profiling to 'first_rebalance.prof'
   wall-time delta: 16.41s

The driver script I used is available here:
https://gist.github.com/adb982aec6f0709f1273

Change-Id: I17e270acb12b5e4d4bbb1e34d8867dea90678961