Add DeleteFunc

This shares DeletePrefixs's implementation.

layeredcache.go

@@ -154,6 +154,11 @@ func (c *LayeredCache) DeletePrefix(primary, prefix string) int {
 	return c.bucket(primary).deletePrefix(primary, prefix, c.deletables)
 }
 
+// Deletes all items that share the same primary key and where the matches func evaluates to true.
+func (c *LayeredCache) DeleteFunc(primary string, matches func(key string, item interface{}) bool) int {
+	return c.bucket(primary).deleteFunc(primary, matches, c.deletables)
+}
+
 //this isn't thread safe. It's meant to be called from non-concurrent tests
 func (c *LayeredCache) Clear() {
 	for _, bucket := range c.buckets {