Add new nodestore benchmarks and a new RocksDB backend for comparative testing.

Created benchmark files are now correctly deleted after the benchmark completes and the random node generation should now be Visual Studio friendly. Here's some more information on why it didn't work previously:
http://connect.microsoft.com/VisualStudio/feedbackdetail/view/856484/std-uniform-int-distribution-does-not-like-char

Reviewers: @vinniefalco  @nbougalis