GODT-1431 Prevent watcher when not using disk on cache

- change: Rename Cacher -> MsgCachePool
- change: Do not run watcher when using memory cache
- add: Allow to cancel cacher jobs (added context)
- change: Fix behavior on context cancel (was causing no internet)

internal/store/user_message.go

@@ -304,7 +304,7 @@ func (store *Store) createOrUpdateMessagesEvent(msgs []*pmapi.Message) error { /
 
 	// Notify the cacher that it should start caching messages.
 	for _, msg := range msgs {
-		store.cacher.newJob(msg.ID)
+		store.msgCachePool.newJob(msg.ID)
 	}
 
 	return nil