discovery: use source of ann upon confirmed channel ann batch
discovery: use source of ann upon confirmed channel ann batch

We do this instead of using the source of the AnnounceSignatures
message, as we filter out the source when broadcasting any
announcements, leading to the remote node not receiving our channel
update. Note that this is done more for the sake of correctness and to
address a flake within the integration tests, as channel updates are
sent directly and reliably to channel counterparts.

aakselrod/lnd,aakselrod/lnd