Consider counterparty commitment tx fees when assembling a route

Consider counterparty commitment tx fees when assembling a route

When calculating the amount available to send for the next HTLC, if
we over-count we may create routes which are not actually usable.

Historically this has been an issue, which we resolve over a few
commits.

Here we consider whether one additional HTLC's commitment tx fees
would result in the counterparty's commitment tx fees being greater
than the reserve we've picked for them and, if so, limit our next
HTLC value to only include dust HTLCs.

We also add some testing when sending to ensure that send failures
are accounted for in our balance calculations.

This, and the previous few commits, fixes #1126.