A write transaction frees pages which its new snapshot cannot see.
A later writer will overwrite them, when no *known* readers can see
them either. But with MDB_NOLOCK, writers don't know about old
readers and might overwrite pages which old readers can see.
Last snapshot is never overwritten. So readers which did begin/renew
after latest commit(write txn) are safe from txn_begin(write txn).
The same with the commit of the write txn before that. I think.
MDB keeps the last two snapshots in the metapages.