Persistent cache: Improvements based on feedback

Read entire cache dir when initializing a persistent tile cache
Obey cache size with persistence enabled
Fix potential NPE in FileSystemTileCache#containsKey() with persistence on
Compatibility constructors for apps developed for 0.5.0
Use file timestamp to determine age of map file
Rename TTL to TimeToLive in all identifiers
Comment for default TTL
Change Mapnik default TTL to 8279 sec and add comment

Signed-off-by: mvglasow <michael -at- vonglasow.com>