watchtower: add sessionID index

In this commit, a new session-ID index is added to the tower client db
with the help of a migration. This index holds a mapping from a
db-assigned-ID (a uint64 encoded using BigSize encoding) to real session
ID (33 bytes). This mapping will help us save space in future when
persisting references to sessions.

watchtower/wtdb/log.go

@@ -8,6 +8,7 @@ import (
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration3"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration4"
 	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration5"
+	"github.com/lightningnetwork/lnd/watchtower/wtdb/migration6"
 )
 
 // log is a logger that is initialized with no output filters.  This
@@ -36,6 +37,7 @@ func UseLogger(logger btclog.Logger) {
 	migration3.UseLogger(logger)
 	migration4.UseLogger(logger)
 	migration5.UseLogger(logger)
+	migration6.UseLogger(logger)
 }
 
 // logClosure is used to provide a closure over expensive logging operations so