journal: truncate archived journals

journal: truncate archived journals

Journal files have space allocated in 8MiB-aligned increments. This
can add up to substantial wasted space as many archived journals
accumulate without using all the allocated space. Let's truncate
journal files to their actually used size when archiving them to
reclaim this unused space.

As the mmap cache is not thread-safe, we can't call
journal_file_move_to_object() from the offline thread. Instead,
we use journal_file_read_object() which doesn't rely on the mmap
cache.