genPOI: use filter functions on the fly

Instead of reading all Players and especially Entities and TileEntities
into RAM and than applying the filter functions onto them, filter
functions are used after each parsed chunk to get only the markers of
this chunk and not all Entities.

This reduced the RAM usage dramatically:
On a 233 MB map over 200 MB of RAM was used before.
Using these changes (and only having two filter functions for signs) not
more than 36 MB of RAM was used.