breacharbiter: filters outputs for dust outputs before sweeping
breacharbiter: filters outputs for dust outputs before sweeping

  This commit refactors the breach arbiter such that it ignores
  commitment values below the remote party's dust limit when trying
  to sweep funds after a channel breach. The wallet is now
  permitted to pass nil sign descriptors for commitment outputs,
  which are then ignored by the arbiter.

  All non-dust outputs are accumulated into a single slice of
  outputs inside the breach arbiter to simplify the internals.
  Doing so permitted a work flow that reduces the total number
  of allocations made while processing breaches.

  The SpendableOutputs is also expanded to include public methods
  to access the witness type and sign descriptors, which was useful
  in refactoring the craft justice transaction logic.

LightningNetwork/lnd,aakselrod/lnd,aakselrod/lnd,LightningNetwork/lnd