Don't try to write past end of file

Do not overwrite loose pages' mp_pgno with mp_next.
mdb_page_flush() will use mp_pgno to rebuild dirty_list.

Try to exit cleanly to avoid leaving stale readers. Not
a critical issue since mdb_reader_check can take care of
them, but still cleaner.

Require that the page is dirty in the current txn,
not a parent txn.

Rename MDB -> LMDB
Integrate tool manpages

Just use srand/rand, we don't care about the quality
of the random numbers, we just want some data.

Don't try this when working on the FreeDB. Again,
too much hassle to unkink the recursions...

Only loosen src page if it's actually dirty.
Refactor page refs in page_merge.

Pages that were dirtied and deleted in the same txn should be
reused, instead of consuming freeDB pages.

Similar to prev commit for mdb_node_move, the call of
mdb_page_serch_lowest() was leaving csrc unusable.

Was leaving stale info in dst cursor

Use num*20, chosen from empirical testing

Was skipping the overwrite in some cases, if the cmp function
said the new and old already matched. We should always perform
the overwrite anyway, since the cmp function may only be doing
a prefix compare and the data may actually be different.

added by mistake in 17c09fa4

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.