Consider dust exposure when assembling a route

Consider dust exposure 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 how much adding one additional (dust) HTLC would
impact our total dust exposure, setting the new next-HTLC-minimum
field to require HTLCs be non-dust if required or set our next-HTLC
maximum if we cannot send a dust HTLC but do have some additional
exposure remaining.

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

Fixes #2252.