Add a TODO
Use a lock in the fake logger

So that the threads don't write over each other.
This sill leaves the issue of writing over messages
coming from the main thread, but this will most probably
not be cleanly resolved until the whole modRana uses proper logging.
Add a module with custom queues

Currently has a custom item leaking LiFo Queue and backported priority queue.
Add a custom thread pool implementations

Currently we have two thread pool implementations differentiated
by the work queue structure they use - one of them uses a priority queue,
which makes it possible to shut it down either immediately
(even if there are work item pending) or once all work is complete.
The  other thread pool uses a stack so that it can handle recently
added. The stack can also "leak" the bottom most item if it becomes full.
The item is returned to the caller of the submit method.

Both stacks also support discarding requests that were in the queue for
longer than a given timeout.
Add a module for thread pool based tile downloading
Use proper thread pool for tile downloading & other tweaks

While the main change is that now instead of starting a new thread for
every tile download a proper thread pool with a work queue
(LiFo in this case) is used, there are also other changes:
* general code cleanup
* newly visible tiles should now should trigger only one download request
* idle tile loader has been turned off as redundant
* tiles should now properly follow loading->waiting->downloading->loaded|error|fatal error
* tile download manager has been renamed and now also handles loading
  of locally available requested tiles to memory cache
* the circular queue & thread conditions are no longer used as the
  thread pool provides more robust alternatives