Reduce lock contention in Cache.WriteMulti

A write-lock was taken the whole time, but we only need the write
lock at the end.

tsdb/engine/tsm1/cache.go

@@ -103,24 +103,26 @@ func (c *Cache) Write(key string, values []Value) error {
 // WriteMulti writes the map of keys and associated values to the cache. This function is goroutine-safe.
 // It returns an error if the cache has exceeded its max size.
 func (c *Cache) WriteMulti(values map[string][]Value) error {
-	c.mu.Lock()
-	defer c.mu.Unlock()
-
 	totalSz := 0
 	for _, v := range values {
 		totalSz += Values(v).Size()
 	}
 
 	// Enough room in the cache?
+	c.mu.RLock()
 	newSize := c.size + uint64(totalSz)
 	if c.maxSize > 0 && newSize+c.snapshotsSize > c.maxSize {
+		c.mu.RUnlock()
 		return ErrCacheMemoryExceeded
 	}
+	c.mu.RUnlock()
 
+	c.mu.Lock()
 	for k, v := range values {
 		c.write(k, v)
 	}
 	c.size = newSize
+	c.mu.Unlock()
 
 	return nil
 }