Use std::queue backed by std::deque instead of list because it does less
allocations which still having O(1) push and pop operations.

Also store trx_purge_rec_t directly, because its only 16 bytes and allocating
it is to wasteful. This should be faster.

purge_node_t::purge_node_t: container is already empty after creation

purge_node_t::end(): replace clearing container with assertion that it's clear