Instead, return a Duration and let the user do the work of waiting.
This is one of only a handful of steps to make us
mostly-syscall-free, at least enough to run in WASM according to
elichai.
This fixes a rather subtle case handling RAAs when we don't
generate a response due to a previous monitor update failure, but
would otherwise send a CS response. We need to still set
AwaitingRemoteRevoke on the channl in question, but previously did
not. Found by chanmon_fail_consistency fuzz test with the failing
test converted and added manually.
