Consider HTLC in-flight count limits when assembling a route

Consider HTLC in-flight count limits 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 the number of in-flight HTLCs which we are allowed
to push towards a counterparty at once, setting the available
balance to zero if we cannot push any further HTLCs.

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