Remove mdb data races. Use (txnid_t)-1 as "no ID".
Avoid race between numreaders++ and reading numreaders at cleanup. Make the un-mutexed reset of reader table entry, atomic: Reset mr_pid only. Instead check mr_pid != 0 in mdb_page_alloc()'s scan for readers. (txnid_t)-1 as "no ID"-mark avoids a check for mr_txnid != 0. The scan can stop when seeing an old reader.