Remove legacy logging in AbstractCell

AbstractCell offered a logging system that predates per-cell log level
adjustments. There is no longer any need for that logging system and
this patch replaces all uses of it with a regular static per-class
Logger instance.

Target: trunk
Require-notes: no
Require-book: no
Acked-by: Paul Millar <paul.millar@desy.de>
Patch: http://rb.dcache.org/r/6279/