> Can't use a DBI from an aborted child txn, unlike aborted
Well, closing the DBI in mdb_txn_abort() caused problems. A fix
in the opposite direction is easy, if we want DBIs to work that
way: mdb_env_open() can open the DBI in any parent txns too.
To get more consistent behavior, if that wasn't clear: Any parent
txn will receive the DBI independently of commit/abort, just like
the MDB_env. Future children of a preexisting txn still differ,
which is probably for the best. Changing that would take locking.