watchtower+lnrpc: remove AckedUpdates from ClientSession struct

In this commit, we start making use of the new ListClientSession
functional options added in the previous commit. We use the functional
options in order to calculate the max commit heights per channel on the
construction of the tower client. We also use the options to count the
total number of acked and committed updates. With this commit, we are
also able to completely remove the AckedUpdates member of the
ClientSession since it is no longer used anywhere in the code.

watchtower/wtdb/client_session.go

@@ -47,13 +47,6 @@ type ClientSession struct {
 	// insertion and retrieval.
 	CommittedUpdates []CommittedUpdate
 
-	// AckedUpdates is a map from sequence number to backup id to record
-	// which revoked states were uploaded via this session.
-	//
-	// NOTE: This map is serialized in it's own bucket, separate from the
-	// body of the ClientSession.
-	AckedUpdates map[uint16]BackupID
-
 	// Tower holds the pubkey and address of the watchtower.
 	//
 	// NOTE: This value is not serialized. It is recovered by looking up the