journal-file: refactor journal_file_rotate()

journal-file: refactor journal_file_rotate()

Let's split the function in three: the part where we archive the old
file into journal_file_archive(), and the part where we initiate the
deferred closing into journal_file_initiate_close().
journal_file_rotate() then simply becomes a wrapper around these two
calls, and the opening of the new journal file.

This useful so that we can archive journal files without having to open
new ones, i.e. to do only the archival part of the rotation, without the
rotation part.