watchtowers: handle closable sessions

Add a routine to the tower client that informs towers of sessions they
can delete and also deletes any info about the session from the client
DB.

sample-lnd.conf

@@ -997,6 +997,12 @@ litecoin.node=ltcd
 ; supported at this time, if none are provided the tower will not be enabled.
 ; wtclient.private-tower-uris=
 
+; The range over which to choose a random number of blocks to wait after the
+; last channel of a session is closed before sending the DeleteSession message
+; to the tower server. The default is currently 288. Note that setting this to
+; a lower value will result in faster session cleanup _but_ that this comes
+; along with reduced privacy from the tower server.
+; wtclient.session-close-range=10
 
 [healthcheck]