CASTOR developers can now choose to use the simple log::write()
"singleton" approach to logging as used in stagerd, vdqmd, etc..

CASTOR developers wishing to unit test their logs (a necessity
for the data transfer session of tapeserverd) can equally choose
to use the logger as an object implementing the interface
class castor::log::Logger.

The goal of this commit and others that will come from
Sebastien is for both logging approaches to use the same
underlying code.