BibSched: enhanced write_message()

• Enhances write_message() function to allow for lazy printing of messages. When calling write_message(msg), msg can now be a callable, that is executed only if the necessary verbosity level is reached (thus allowing printing detailed information without performance penalties).

Signed-off-by: Samuele Kaplun <samuele.kaplun@cern.ch>
Reviewed-by: Tibor Simko <tibor.simko@cern.ch>