Use cdb library and provide a back-compat layer

The new cdb library is pulled in via composer. Since the
library uses namespaces, a backwards-compatability layer
is provided for the old class names:
* CdbReader
* CdbWriter
* CdbException

The PHP/DBA-specific classes should never have been used directly.

Depends on I98302bdf1 in mediawiki/vendor

Change-Id: I39549ac8540b262cf91f7d1830d36327afb3033d