This makes the response time way more snappy when the maps thread
is updating its cache.
* Move the `MapsHandle` type to the `maps` module
* SWitch to using the standard library mutex
* Split refresh methods into retrieval methods that don't need the lock
and check timestamp & update methods that only need it shortly
* Introduce the `MapsRefresh` trait and implement it for `MapsHandle`
* Reorder some methods for clarity
* Small documentation fixes
* Replace the lazy `once_cell` by a maps handle type
* Use Rocket's managed state to manage a handle
* Ensure that the handlers have access to it
* Pass another handle to the maps updater loop
* Try to keep the lock as short as possible
Still, long downloads block the lock. Add a FIXME to refactor this
so the lock is only taken when updating the maps fields.
Paul van Tilburg