Move IRC's disk logging to thalamus/channel_log.rs as shared
functions: append_disk_log() and load_disk_log(). Any daemon
can use them — opt-in, not mandatory (tmux won't use them).
load_disk_log() populates a ChannelLog from disk on startup,
so history survives daemon restarts.
IRC daemon now uses the shared functions.
Co-Authored-By: Kent Overstreet <kent.overstreet@linux.dev>