Clean up surface of interfaces

To avoid the leakage of store handling and the advancing in time the interfaces which sole purpose is the handling of internal doozerd (through cotterpin) revisions have been removed. All assumptions about the state of the receiver and its snapshot for conflict free functionality have been removed and every method is responsible to work with an appropriate snapshot. The cotterpin snapshotable interface is still satisfied by all types exposed by glimpse, to keep functionality which helps to iterate quickly like GetSnapshotables and keep access to the types Snapshot consistent.

* Remove Join methods
* Do not expose FastForward on directory