Post-push fix: purge queue array can't be fixed size, because the elements
of the array is the analogue of undo logs, which must be processed in
the order of transaction commits, and the array can contain more
elements, than trx_sys.rseg_array. Also it's necessary to maintain
min-heap property by the trx_no of transaction, which produced the first
non-purged undo log in all rsegs. That's why the element of purge queue
aray must contain not only trx_sys.rseg_array index, but also trx_no of
committed transacion, i.e. the pair (trx_no, trx_sys.rseg_array index),
which is encoded as uint64_t((trx_no << 8) | (trx_sys.rseg_array index)).

Reviewed by: Marko Mäkelä