Due to commit 18a07eb7

With mdb_cursor_put(new key, data item big enough to need a sub-DB):
- Track cursors as when adding other nodes.  The "!do_sub" is now
  unneeded.  Before ITS#7861 it distinguished inserting key from data.
- Set MDB_TXN_ERROR if an empty sub-DB was created.  I.e. adding
  key:subDB succeeded, but inserting the data item failed.

Update the state before looping, so the key is not duplicated.
Broke in 4c8f5761 (mostly).

mdb_cursor_del(): Fix md_entries with MDB_NODUPDATA.
mdb_cursor_put(): Check in xcursor whether an item got inserted. Count
data instead of keys after 13d6d59c.

Since non-NULL key must be provided now.

Shift the rest of the cursor stack as needed

Make sure to propagate rebalance results all the
way back to original caller's cursor.

Fixup cursor state after node_move/page_merge

Fix md_entries count, test for insert on md_entries increment

Note MDB_CURRENT/MDB_DUPSORT requirement

Clarify MDB_CURRENT key usage, simplify freelist_save

This reverts commit 4e954f8a.
Caused ITS#7828.

This reverts commit 6bd34d05.

It does not return MDB error codes.

Prepares for the next commits.

Factor txn/DBI-checks out to TXN_DBI_EXIST().
mdb_audit(): Skip closed DBIs.
mdb_cursor_renew(), mdb_stat(): Check DBI and txn.
mdb_cursor_count(): Check txn.
mdb_dbi_flags(): Check DBI.

Replace env flag MDB_LIVE_READER with field me_live_reader.

They're meant to be read and studied, not run.

This case didn't come up in testing. Needs finer grained test.

Was leaving cursor pointed at wrong page if the newindx was
in the right sibling page, and the parent was also split.

Ignore key, broken by 5bda3565
and some older code.  Document and clarify MDB_CURRENT usage.

Also affects non-MDB_CURRENT put() with empty data and a key
which matches by the mdb_set_compare function but not by memcmp.