[journal] add support for aligned appends
Journal entries are normally appended to the the end of the journal object. Unaligned appends can result in between 2 - 3 writes within bluestore (depending on the append size and min alloc size). One potential solution would be to support a new "journal::append" cls method that could automatically re-align the write based on the current journal size. This would also require supporting a new "pad" journal event that can skip X number of bytes in the replay stream.