Fix deadlock in metastore

The interaction of continuous query service, the meta-store loading
and initializing raft state, and syncing node info could cause a
deadlock in some instances.  There was an extra read-lock taken by isLeader()
when it already had a read-lock.  Removing this extra lock fixes the startup
deadlock.

Fixes influxdata#3607

(Cherry-picked from 0286a3e)

meta/state.go

@@ -286,8 +286,6 @@ func (r *localRaft) leader() string {
 }
 
 func (r *localRaft) isLeader() bool {
-	r.store.mu.RLock()
-	defer r.store.mu.RUnlock()
 	if r.raft == nil {
 		return false
 	}