Fix #943: don't try to take 2 snapshots at a time

The `TakeSnapshot()` function in _vendor/raft/server.go was failing at `s.stateMachine.Save()` and exiting without resetting `s.pendingShapshot = nil`.  That left the raft server in an invalid state.

`s.stateMachine.Save()` was failing with the error message "gob: encodeReflectValue: nil element", which comes from the `Save()` function in cluster/cluster_configuration.go.  This is a separate issue.
Without the lock, influxdb attempts to take multiple snapshots at the same time.