journald: add debug logs around offlining/archiving/rotating/varlink operations

It is not easy to understand what happens to a journal file
even with debug logs enabled. Add more dbg messages around operations
started by users to make it possible to follow the flow of operations.

src/libsystemd/sd-journal/journal-file.c

@@ -197,10 +197,14 @@ int journal_file_set_offline_thread_join(JournalFile *f) {
         if (f->offline_state == OFFLINE_JOINED)
                 return 0;
 
+        log_debug("Joining journal offlining thread for %s.", f->path);
+
         r = pthread_join(f->offline_thread, NULL);
         if (r)
                 return -r;
 
+        log_debug("Journal offlining thread for %s joined.", f->path);
+
         f->offline_state = OFFLINE_JOINED;
 
         if (mmap_cache_fd_got_sigbus(f->cache_fd))