api.go
func (r *Raft) Snapshot() SnapshotFuture {
	future := &userSnapshotFuture{}
	future.init()
	select {
	case r.userSnapshotCh <- future:
		return future
	case <-r.shutdownCh:
		future.respond(ErrRaftShutdown)
		return future
	}
}
func (r *Raft) Snapshot() SnapshotFuture {
	future := &userSnapshotFuture{}
	future.init()
	select {
	case r.userSnapshotCh <- future:
		return future
	case <-r.shutdownCh:
		future.respond(ErrRaftShutdown)
		return future
	}
}
// Snapshot is used to manually force Raft to take a snapshot. Returns a future
// that can be used to block until complete, and that contains a function that
// can be used to open the snapshot.