Fix fee check when accepting inbound HTLCs

[TheBlueMatt]

We currently check that new inbound HTLCs meet reserve requirements just fine, but we don't check that they meet reserve requirements once we consider that the sender may be the initiator and need to pay fees.

There are a few aspects to this:
a) we need to check it for inbound channels (I think TheBlueMatt@7c96416 does this correctly)
b) we need to check it for outbound channels before we send HTLCs, so that we don't get closed on,
c) we need to write tests for both cases,
d) we should make sure the chanmon_consistency fuzz target can hit this case, it may need new "actions" in it to send low-value HTLCs to better hit edge cases.