Move network sync to seperate worker thread

[JackNoordhuis]

Network sync synchronization currently hogs the workers and causes other async tasks to be completed slowly, this only worsens the longer the server is online.

[JackNoordhuis]

This issue is somewhat mitigated by 675dceb but still takes up a large chunk of time in comparison to other requests. An even more 'thread friendly' version of the network map implementation will be required if network sync is to be moved to its own worker thread.