campaignd: Refactor request handling blocks into separate methods

These methods are tried and run in sequence by looking them up from a
request handler registry maintained as part of the server object. Since
every campaignd request is a single WML node with an identifying name, I
feel this approach makes things more readable than the previous massive
deeply-nested if-else-if chain approach, but that might be just me.

(Backported c164cb2 from master.)

Conflicts:
	src/campaign_server/campaign_server.cpp