Add async-mode katcp server tests, and fix breakage

Some rework was also done in terms of how request futures are handled, meaning that request handlers can now return a future that will be passed on the KATPServer, and is also joined onto MessageRequestThread's futures using chain_future(). Making some async standard request handlers work in both async and MessageRequestThread mode neccessitated those changes.

Apart from passing own package tests, it was also tested against katproxy, katnodeman and katcore