watchtower/wtclient/client: use existing session with same TxPolicy

This commit modifies the client's filtering when selecting from existing
sessions. The new logic compares the configured TxPolicy with the
TxPolicy of the candidate sessions, which has the effect of ignoring
operational parameters such as MaxUpdates. Prior, changing MaxUpdates
would cause the client to request a new session even if it had perfectly
good slots available in a policy with an equal TxPolicy.

watchtower/wtclient/client.go

@@ -519,9 +519,10 @@ func (c *TowerClient) nextSessionQueue() *sessionQueue {
 		delete(c.candidateSessions, id)
 
 		// Skip any sessions with policies that don't match the current
-		// configuration. These can be used again if the client changes
-		// their configuration back.
-		if sessionInfo.Policy != c.cfg.Policy {
+		// TxPolicy, as they would result in different justice
+		// transactions from what is requested. These can be used again
+		// if the client changes their configuration and restarting.
+		if sessionInfo.Policy.TxPolicy != c.cfg.Policy.TxPolicy {
 			continue
 		}