watchtower/wtdb: add tower-to-session index entry for all towers

In this commit, a small migration is added to the watchtower client DB
to ensure that there is an entry in the towerID-to-sessionID index for
all towers in the db regardless of if they have sessions or not. This is
required as a follow up to migration 1 since that migration only created
entries in the index for towers that had associated sessions which would
lead to "tower not found" errors on start up.

docs/release-notes/release-notes-0.16.0.md

@@ -406,7 +406,10 @@ in the lnwire package](https://github.com/lightningnetwork/lnd/pull/7303)
   speed of listing sessions for a particular tower ID](
   https://github.com/lightningnetwork/lnd/pull/6972). This PR also ensures a 
   closer coupling of Towers and Sessions and ensures that a session cannot be
-  added if the tower it is referring to does not exist.
+  added if the tower it is referring to does not exist. A [follow-up migration 
+  was added](https://github.com/lightningnetwork/lnd/pull/7491) to ensure that 
+  entries are added to the new index for _all_ towers in the db, including those
+  for which there are not yet associated sessions. 
 
 * [Remove `AckedUpdates` & `CommittedUpdates` from the `ClientSession`
   struct](https://github.com/lightningnetwork/lnd/pull/6928) in order to