In this commit, we modify the existing logic that defaults to implicit
commitment type negotiation to support explicit negotiation if the new
feature bit is set. This change allows us to ditch the notion of a
"default" commitment type, as we'll now use feature bits to signal our
understanding of a commiment type, but allow peers to select which
commitment type they actually wish to use.
In addition, this explicit negotiation removes the need for using the
required bit of any commitment types. Instead, if an implementation
wishes to no longer support a commitment type, they should simply stop
advertising the optional bit.
This field will be examined later down the stack along with the set of
feature bits to determine if explicit channel commitment type
negotiation is possible or not.
In order to be consistent with other sub systems an error is now
returned from the Stop functions.
This also allows writing a generic cleanup mechanism to stop all
sub systems in case of a failure.
This commit duplicates the utxonursery's writeOutpoint function
in the funding package so that when the rest of the fundingmanager
code is moved, it can use the WriteOutpoint function for its
channel opening state data.
