Bugfix: add RemoteCommit.toString()

This allows the logger in ClusterUtils.sendCommitInfoToOthers to be able to
abstract itself from what type of RemoteCommit we're using.  When the
SpeculativeRemoteCommit was added it had introduced a bug here.
Merge pull request #152 from smmf/fix/log-remote-commit

Bugfix: add RemoteCommit.toString()