[crossfire] Map cache

[Anton Oussik]

This is an idea I had for a while now, but never had time to
implement. If someone has some spare time and wants something to code
something for CF they can try this.

When the server comes across maps containing large amounts of items it
takes a long time to load. While it is loading the whole server
freezes for other players, and on very large maps (like the scorn sale
shop or some apartments) this can take in excess of 10 seconds on
metalforge.

A solution I propose is to pre-load large maps and keep them around in
memory in case they are needed. Whilst this option would improve the
performance of the server like metalforge, it would cause servers with
small physical memory size to start swapping the cached maps, and it
would then not be beneficial. Therefore the map caching should be
optional and off by default if implemented.

Casper