peer: add additional comments around new channel close workflow, minor fixes
peer: add additional comments around new channel close workflow, minor fixes

This commit adds a set of additional comments around the new channel
closure workflow and also includes two minor fixes:
  * The error when parsing a signature previously wasn’t checked and is
    now.
  * As a result, we should only track the new signature iff it parses
    correctly and we agree to the details as specified w.r.t to the fee
    for the final closing transaction.

Additionally, as set of TODO’s has been added detailing the additional
work that needs to be done before the closing workflow is fully
compliant with the specification.

aakselrod/lnd,LightningNetwork/lnd,AndrewSamokhvalov/lnd,AndrewSamokhvalov/lnd,LightningNetwork/lnd,aakselrod/lnd,ddustin/lnd,ddustin/lnd