journal: fix already offline check and thread leak (#2810)

journal: fix already offline check and thread leak (#2810)

Early in journal_file_set_offline() f->header->state is tested to see if
it's != STATE_ONLINE, and since there's no need to do anything if the
journal isn't online, the function simply returned here.

Since moving part of the offlining process to a separate thread, there
are two problems here:

1. We can't simply check f->header->state, because if there is an
offline thread active it may modify f->header->state.

2. Even if the journal is deemed offline, the thread responsible may
still need joining, so a bare return may leak the thread's resources
like its stack.

To address #1, the helper journal_file_is_offlining() is called prior to
accessing f->header->state.

If journal_file_is_offlining() returns true, f->header->state isn't even
checked, because an offlining journal is obviously online, and we'll
just continue with the normal set offline code path.

If journal_file_is_offlining() returns false, then it's safe to check
f->header->state, because the offline_state is beyond the point of
modifying f->header->state, and there's a memory barrier in the helper.

If we find f->header->state is != STATE_ONLINE, then we call the
idempotent journal_file_set_offline_thread_join() on the way out of the
function, to join a potential lingering offline thread.