Add solo mode without consensus enabled

This is an attempt to run doozerd in a single node scenario. By
introducing the solo package we mimic the functionality of the peer
- hookking together store, server and gc - while leaving out the
consensus.Manager entirley. To avoid migration pains the mode can be
enabled with the `-solo` flag.
Merge pull request #11 from soundcloud/consensus-cleanup

Implement solo mode without consensus enabled