Drop StringKeyCache mutex

The StringKeyCache is now only used inside a connection. It's assumed that connections are not used concurrently with multiple queries.

src/db/string_key_cache.cr

@@ -1,28 +1,21 @@
 module DB
   class StringKeyCache(T)
     @cache = {} of String => T
-    @mutex = Mutex.new
 
     def fetch(key : String) : T
-      @mutex.synchronize do
       value = @cache.fetch(key, nil)
       value = @cache[key] = yield unless value
       value
     end
-    end
 
     def each_value
-      @mutex.synchronize do
       @cache.each do |_, value|
         yield value
       end
     end
-    end
 
     def clear
-      @mutex.synchronize do
       @cache.clear
     end
   end
-  end
 end