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.

watchtower/wtclient/interface.go

@@ -68,6 +68,14 @@ type DB interface {
 	FetchSessionCommittedUpdates(id *wtdb.SessionID) (
 		[]wtdb.CommittedUpdate, error)
 
+	// IsAcked returns true if the given backup has been backed up using
+	// the given session.
+	IsAcked(id *wtdb.SessionID, backupID *wtdb.BackupID) (bool, error)
+
+	// NumAckedUpdates returns the number of backups that have been
+	// successfully backed up using the given session.
+	NumAckedUpdates(id *wtdb.SessionID) (uint64, error)
+
 	// FetchChanSummaries loads a mapping from all registered channels to
 	// their channel summaries.
 	FetchChanSummaries() (wtdb.ChannelSummaries, error)