[performance] cache v2 filter keyword regular expressions (#2903)

* add caching of filterkeyword regular expressions

* formatting

* fix WholeWord nil check

internal/cache/db.go

@@ -531,6 +531,11 @@ func (c *Caches) initFilterKeyword() {
 		// See internal/db/bundb/filter.go.
 		filterKeyword2.Filter = nil
 
+		// We specifically DO NOT unset
+		// the regexp field here, as any
+		// regexp.Regexp instance is safe
+		// for concurrent access.
+
 		return filterKeyword2
 	}