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/wtdb/log.go

@@ -6,6 +6,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration1"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration2"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -32,6 +33,7 @@ func UseLogger(logger btclog.Logger) {
 	migration1.UseLogger(logger)
 	migration2.UseLogger(logger)
 	migration3.UseLogger(logger)
+	migration4.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so