Merge 1.7.1 fixes to develop

[engelsanchez]

This is a reminder to merge the Bitcask changes going into the Riak 2.0.1 release, which were done in the Bitcask 1.7 branch, onto develop. So far we have:

• Race makes opening Bitcask dir impossible #187
• Bitcask may get stuck due to caching port  #188
• Remove expired on get race #191

I will prepare a staging branch to merge everything in one go after the release.