[webrunner]: use latest key in equeue.db to get last backup date

in some webrunner, there are 2 keys in equeue.db:

```
>>> print(db.keys())
['/srv/slapgrid/slappart18/bin//runner-importer', '/srv/slapgrid/slappart18/bin/runner-importer']
```

We suppose that the 2nd key was used after the first key so we take the
timestamp of the last key.

/reviewed-on nexedi/slapos!578

stack/resilient/buildout.hash.cfg

@@ -46,7 +46,7 @@ md5sum = d21472f0e58f928fb827f2cbf22c4d4a
 
 [resilient-web-takeover-cgi-script-download]
 filename = resilient-web-takeover-cgi-script.py.in
-md5sum = 9d258d41eeef66f44f361adaa15cbd71
+md5sum = 60d4912fdf5e8dafaba9d9f333aa9e36
 
 [template-wrapper]
 filename = templates/wrapper.in

stack/resilient/resilient-web-takeover-cgi-script.py.in

@@ -42,7 +42,7 @@ def getLatestBackupDate():
   if not db.keys():
     result = False
   else:
-    last_backup = db[db.keys()[0]]
+    last_backup = db[db.keys()[-1]]
     for callback in db.keys():
       timestamp = float(db[callback])
       if timestamp < last_backup: