Verify Bitcask tombstone 2 upgrade

Verify that upgrading Riak with Bitcask to 2.0 or later will trigger
an upgrade mechanism that will end up merging all existing bitcask
files.  This is necessary so that old style tombstones are reaped,
which might otherwise stay around for a very long time. This version
writes tombstones that can be safely dropped during a merge. Bitcask
could resurrect old values easily when reaping tombstones during a
 partial merge if a restart happened later.

This tests these PRs:

* https://github.com/basho/riak_kv/pull/925
* https://github.com/basho/bitcask/pull/156