Don't lock the cache while adding entries

Entries have their own locking so the cache doesn't need to be lock
when adding to them.

tsdb/engine/tsm1/cache.go

@@ -185,10 +185,10 @@ func (c *Cache) WriteMulti(values map[string][]Value) error {
 	}
 	c.mu.RUnlock()
 
-	c.mu.Lock()
 	for k, v := range values {
-		c.write(k, v)
+		c.entry(k).add(v)
 	}
+	c.mu.Lock()
 	c.size = newSize
 	c.mu.Unlock()
 
@@ -416,6 +416,17 @@ func (c *Cache) write(key string, values []Value) {
 	e.add(values)
 }
 
+func (c *Cache) entry(key string) *entry {
+	c.mu.Lock()
+	e, ok := c.store[key]
+	if !ok {
+		e = newEntry()
+		c.store[key] = e
+	}
+	c.mu.Unlock()
+	return e
+}
+
 // CacheLoader processes a set of WAL segment files, and loads a cache with the data
 // contained within those files.  Processing of the supplied files take place in the
 // order they exist in the files slice.