When users set `gossip.ban-threshold` to 0, it's now treated as setting
the ban score to max uint64, which effectively disables the banning. We
still want to record the peer's ban score in case we need it for future
debugging.
This commit introduces a ban manager that marks peers as banned if
they send too many invalid channel announcements to us. Expired
entries are purged after a certain period of time (currently 48 hours).
Elle Mouton