Fix PB connection pool (pbc_pool_master) leaks

This PR addresses two cases described in #985 .

The specfic requests are in the comment of the above issue.
https://github.com/basho/riak_cs/issues/985#issuecomment-57976044



I would merge the fix in `riak_cs_multibag`, create a new tag on that repo, and update this prior to merging. Otherwise you're introducing nondeterminism into the build.