Yubico

* Fix bug in ykval-checksum-clients.php when used with PostgreSQL.

* Add ykval-checksum-clients.php, see doc/SyncMonitor.wiki.

* Minor cleanups and fixes.

* Add ykval-revoke.php service, see doc/RevocationService.wiki.

* Minor cleanups and fixes.

* Major re-design to support a new architecture with replicated
servers.

-------------------

* Fix compatibility issue with Python versions >= 3.0 <= 3.3 22
* Pin ``requests`` dependency to the latest version (2.7) 25 27

Contribution by Wouter van Bommel, Vianney Carel.

* Make sure the query parameters are unquoted when parsing them from the
response. 23

Contribution by Tamás Gulácsi.