route blinding: use all valid routes during path construction iteration instead of the currently capped set

[ellemouton]

Today, if we are constructing set of blinded payment paths to add to an invoice, we:

1. Select up to MaxNumPaths routes to our own node that are considered blinded path candidates.
2. This set is then passed to blindedpath.BuildBlindedPaymentPaths which attempts to use each path to build a valid blinded path
3. If blindedpath.BuildBlindedPaymentPaths finds that it cannot use one of the paths it was given, it skips it and moves on to the next one

This order of operations means that we may run out of paths in blindedpath.BuildBlindedPaymentPaths and we wont go back and try some of the paths that we initially found by ChannelRouter's findBlindedPaths .

What we instead want is for blindedpath.BuildBlindedPaymentPaths to be able to continue with the iteration through possibly all of the routes found in findBlindedPaths until it reaches the max specified by MaxNumPaths.