Add channel_keys dict to NetworkConfig for storing per-channel keys.
Keys are used in KICK rejoin, passed via AUTOJOIN +#channel key syntax,
supported in ADDNETWORK channel_keys= parameter, and propagated through
REHASH. Extract rehash() as reusable async function for SIGHUP reuse.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>