I believe this now implements all the checks listed in BOLT-02 for
update_add_htlc, however, the BOLT is sometimes ambiguous,
and actually the checks listed there IMO are insufficient.
There are still some TODOs, in part because of the above.
we were looking at inconsistent ctns
and we were looking at the wrong subject's ctx
all the FIXMEs and TODOs here will still warrant some attention.
(note that test_DesyncHTLCs was passing incorrectly:
the "assertRaises" was catching a different exception)
note: low R grinding would not have to be duplicated if we trusted the caller
to have done it already (as is the case with the classes in ecc.py), and if
we propagated the choice of "random_k" as part of the nonce_function passed
to libsecp256k1 (which is not currently done)
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)
