channel_log: shared disk logging with round-trip

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>

Cargo.lock

@@ -2593,6 +2593,7 @@ dependencies = [
  "rustls-platform-verifier",
  "serde",
  "serde_json",
+ "serde_urlencoded",
  "sync_wrapper",
  "tokio",
  "tokio-rustls",
@@ -2902,6 +2903,18 @@ dependencies = [
  "zmij",
 ]
 
+[[package]]
+name = "serde_urlencoded"
+version = "0.7.1"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "d3491c14715ca2294c4d6a88f15e84739788c1d030eed8c110436aafdaa2f3fd"
+dependencies = [
+ "form_urlencoded",
+ "itoa",
+ "ryu",
+ "serde",
+]
+
 [[package]]
 name = "sha2"
 version = "0.10.9"