[crossfire] Map cache

[Anton Oussik]

Oh, and I wanted to add to that that when a server goes down, another
server could take over its load, but states of maps and players on the
server would be lost. They could send each other "alive" packets, and
when a server has not replied for a while consider it dead and either
hold election or decide from server loads who should take over the
maps. Also when transferring a player server should be sure the server
they are transferring the player to is there, otherwise report the
exit as closed.

Unique items, player files, and maps would also need to be distributed
form server to server, which raises another problem.