[backend] Refactor bs_publish and bs_repserver notify function into BSNotify::notify

This removes code duplication and adds some clarifications how the
notification mechanism is suppose to work. Obviously the source server
itself should not call BSNotify::notify.

Signed-off-by: Jan Blunck <jblunck@infradead.org>
[backend] Move generate_commit_flist to bs_srcserver

The function is only used in source server.

Signed-off-by: Jan Blunck <jblunck@infradead.org>