multi: allow setting target confs for co-op close

This commit adds a new config option: "--coop-close-target-confs"
which allows a user to override the default target confirmations of 6
that is used to estimate a fee rate to use during a co-op closure
initiated by a remote peer.

sample-lnd.conf

@@ -252,6 +252,11 @@
 ; to better align with your risk tolerance
 ; maxchansize=
 
+; The target number of blocks in which a cooperative close initiated by a remote
+; peer should be confirmed. This target is used to estimate the starting fee
+; rate that will be used during fee negotiation with the peer.
+; coop-close-target-confs=10
+
 ; The default max_htlc applied when opening or accepting channels. This value
 ; limits the number of concurrent HTLCs that the remote party can add to the
 ; commitment. The maximum possible value is 483.