Replace allegro/bigcache with something lighter.

In high frequency environment the allegro bigcache caused significant
memory usage overhead. This commit replaces it with something much lighter.

cache.go

@@ -2,10 +2,10 @@ package gql
 
 func (c *BaseClient) cacheLookup(hash string) []byte {
 	if c.cache.client != nil {
-		g, err := c.cache.client.Get(hash)
-		if err == nil {
+		obj, found := c.cache.client.Get(hash)
+		if found {
 			c.Logger.Debug(c, "Cache hit;", "hash", hash)
-			return g
+			return obj.([]byte)
 		}
 		// if error is not nil it means that hash is not present in cache
 		c.Logger.Debug(c, "Cache miss;", "hash", hash)