More progress in making disk backed queue use alt cache, made it use alt blob_t.
Finished internal_disk_backed_queue_t::push conversion to alt cache.
Made the whole disk backed queue work with alt cache, probably not without bugs though.
Made alt_txn_t constructor handle preceding_txn being NULL.
Made the alt cache support creating a block with specified id (such as SUPERBLOCK_ID).
Made disk_backed_queue call the create version of alt_buf_lock_t for the SUPERBLOCK_ID.