journal: Move offlining logic to journald-file.c

journal: Move offlining logic to journald-file.c

With this change, the logic to write the final tag, emit the final
change notification and to offline the file moves from journal_file_close()
to journald_file_close(). Since all this logic is only executed when
the journal file is writable and all code that writes journal files
already uses journald_file_close() instead of journal_file_close(), this
change should not introduce any changes in behaviour.

Moving the offline related logic to journald-file.c allows us to use
code from src/shared in the offlining logic, more specifically, we can
use the file copying logic from copy.h to fix BTRFS filesystem compression
for journal files when archiving.