multi: migrate towers to use RangeIndex for AckedUpdates

In this commit, a migration is done that takes all the AckedUpdates of
all sessions and stores them in the RangeIndex pattern instead and
deletes the session's old AckedUpdates bucket. All the logic in the code
is also updates in order to write and read from this new structure.

lnrpc/wtclientrpc/wtclient.go

@@ -341,27 +341,27 @@ func constructFunctionalOptions(includeSessions bool) (
 
 	var (
 		opts                  []wtdb.ClientSessionListOption
-		ackCounts             = make(map[wtdb.SessionID]uint16)
 		committedUpdateCounts = make(map[wtdb.SessionID]uint16)
+		ackCounts             = make(map[wtdb.SessionID]uint16)
 	)
 	if !includeSessions {
 		return opts, ackCounts, committedUpdateCounts
 	}
 
-	perAckedUpdate := func(s *wtdb.ClientSession, _ uint16,
-		_ wtdb.BackupID) {
+	perNumAckedUpdates := func(s *wtdb.ClientSession, id lnwire.ChannelID,
+		numUpdates uint16) {
 
-		ackCounts[s.ID]++
+		ackCounts[s.ID] += numUpdates
 	}
 
 	perCommittedUpdate := func(s *wtdb.ClientSession,
-		_ *wtdb.CommittedUpdate) {
+		u *wtdb.CommittedUpdate) {
 
 		committedUpdateCounts[s.ID]++
 	}
 
 	opts = []wtdb.ClientSessionListOption{
-		wtdb.WithPerAckedUpdate(perAckedUpdate),
+		wtdb.WithPerNumAckedUpdates(perNumAckedUpdates),
 		wtdb.WithPerCommittedUpdate(perCommittedUpdate),
 	}
 
@@ -438,7 +438,8 @@ func (c *WatchtowerClient) Policy(ctx context.Context,
 // marshallTower converts a client registered watchtower into its corresponding
 // RPC type.
 func marshallTower(tower *wtclient.RegisteredTower, includeSessions bool,
-	ackCounts, pendingCounts map[wtdb.SessionID]uint16) *Tower {
+	ackCounts map[wtdb.SessionID]uint16,
+	pendingCounts map[wtdb.SessionID]uint16) *Tower {
 
 	rpcAddrs := make([]string, 0, len(tower.Addresses))
 	for _, addr := range tower.Addresses {