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/migration6/log.go

@@ -0,0 +1,14 @@
+package migration6
+
+import (
+	"github.com/btcsuite/btclog"
+)
+
+// log is a logger that is initialized as disabled.  This means the package will
+// not perform any logging by default until a logger is set.
+var log = btclog.Disabled
+
+// UseLogger uses a specified Logger to output package logging info.
+func UseLogger(logger btclog.Logger) {
+	log = logger
+}