watchtower: use bbolt db instead of mock DB for client tests

The watchtower client test framework currently uses a mock version of
the tower client DB. This can lead to bugs if the mock DB works slightly
differently to the actual bbolt DB. So this commit ensures that we only
use the bbolt db for the tower client tests. We also increment the
`waitTime` used in the tests a bit to account for the slightly longer DB
read and write times. Doing this switch resulted in one bug being
caught: we were not removing sessions from the in-memory set on deletion
of the session and so that is fixed here too.

watchtower/wtclient/client.go

@@ -977,6 +977,19 @@ func (c *TowerClient) handleClosableSessions(
 				// and handle it.
 				c.closableSessionQueue.Pop()
 
+				// Stop the session and remove it from the
+				// in-memory set.
+				err := c.activeSessions.StopAndRemove(
+					item.sessionID,
+				)
+				if err != nil {
+					c.log.Errorf("could not remove "+
+						"session(%s) from in-memory "+
+						"set: %v", item.sessionID, err)
+
+					return
+				}
+
 				// Fetch the session from the DB so that we can
 				// extract the Tower info.
 				sess, err := c.cfg.DB.GetClientSession(