The few other cases that used SimpleConfig.get_instance() now
either get passed a config instance, or they try to get a reference
to something else that has a reference to a config.
(see lnsweep, qt/qrcodewidget, qt/qrtextedit)
"current" used to be "oldest_unrevoked"; and pending was "oldest_unrevoked + 1"
but this was very confusing...
so now we have "oldest_unrevoked", "latest", and "next"
where "next" is "latest + 1"
"oldest_unrevoked" and "latest" are either the same or are offset by 1
(but caller should know which one they need)
rm "got_sig_for_next" - it was a redundant sanity check, that really
just complicated things
rm "local_commitment", "remote_commitment", "set_local_commitment",
"set_remote_commitment" - just use "get_latest_commitment" instead
in the old code,
`self.hm.received_in_ctn(self.config[REMOTE].ctn + 1)`
did not really make sense as "received_in_ctn" compares the argument against the LOCAL ctn
