Add missing RUnlock's to m3aggregator locking code (#3446)

These were missed in https://github.com/m3db/m3/pull/3315/files, and were causing hangs for us in prod.

Suspected sequence leading to hangs:

- RLock()
- Missed unlock on shard not owned `if int(shardID) >= len(agg.shards) return errShardNotOwned`
- Lock() from placement change (will never succeed)
- RLock() -- hangs all subsequent times due to pending Lock()

Once threads start hitting the final RLock, the aggregator locks up and stops processing metrics.

RWMutex behavior is consistent with the docs:

```
// If a goroutine holds a RWMutex for reading and another goroutine might
// call Lock, no goroutine should expect to be able to acquire a read lock
// until the initial read lock is released.
```

https://gist.github.com/andrewmains12/fabc257f4552f8e778828a23346d71c3 has a rough reproduction of the above locking sequence.

src/aggregator/aggregator/aggregator.go

@@ -198,13 +198,13 @@ func (agg *aggregator) placementTick() {
 			return
 		}
 
-		agg.RLock()
 		placement, err := agg.placementManager.Placement()
 		if err != nil {
 			m.updateFailures.Inc(1)
 			continue
 		}
 
+		agg.RLock()
 		if !agg.shouldProcessPlacementWithLock(placement) {
 			agg.RUnlock()
 			continue
@@ -394,8 +394,10 @@ func (agg *aggregator) shardFor(id id.RawID) (*aggregatorShard, error) {
 		shardID = agg.shardFn(id, uint32(numShards))
 	}
 
+	// Maintain the rlock as long as we're accessing agg.shards (since it can be mutated otherwise).
 	agg.RLock()
 	if int(shardID) >= len(agg.shards) {
+		agg.RUnlock()
 		return nil, errShardNotOwned
 	}
 	shard := agg.shards[shardID]