mysql connections cache

* Hash table `uri -> Mysql_connection` to have only one connection per mysql server used.
  * Included htable source code from [ccan](http://ccodearchive.net/info/htable.html) libraries
* Improved unit test (unnecessary creation database per individual test, now x2 faster)
  * Tweak debug level of some messages.

work started at https://github.com/Zentyal/openchange/pull/66