Replace all remaining usages of `deadline_timer` with `steady_timer`

[jschmidt-icinga]

boost::asio::deadline_timer has been deprecated for a while and boost::asio::steady_timer is supported in all boost versions we support and already in use in several places on master.

Another issue is that deadline_timer is based on CLOCK_REALTIME instead of CLOCK_MONOTONIC, which may be more appropriate for the way we use these timers (as timeouts and intervals).

Also it's already annoying enough having to deal with two duration representations (double and std::chrono::duration), so getting rid of a third one (boost::posix_time::milliseconds) would be a good thing.