FileWatcher: honor disk cache size
Review Request #119982 - Created Aug. 28, 2014 and submitted
Current code stops are identifying localDir()/maps as dangerous to handle.
This implementation gets a lot simpler, trying to remove oldest map tiles below level 4 to keep at disk cache size value.
The internal weirdness has been kept (delete in batches, define a soft limit...) until some more cleanup is done.
Basic functionnality is here, if you don't want your precious tiles to be dropped you may toggle writeable bit off.