Remove underscore from CdbReader_[DBA|PHP] classes and related file

Also gets rid of some CodeSniffer errors. There are more cases in core
(cache, pool counter, installer, database, load balancer, diff, CSS
Janus, less (argh!), media, parser, revdel, ...), that have class names
with underscores I'd be trying to get rid of later.

Change-Id: I33709c05e597978a5574a445fa43c583cbd7e12b