mdb_txn_commit(child): Give loose pages to parent.
Use a pointer beyond the page header instead of mp_next, so
we will not need to save/restore mp_pgno. This avoids a crash
caused by references to mp_pgno.

Fix in 534dcc72 was in wrong place

Fix mp_lower/mp_upper to exclude PAGEHDRSZ, which means our
max mp_upper is now pagesize - PAGEHDRSZ and won't overflow.
Currently must define MDB_DEVEL to use. Will be default for
LMDB 1.0/OpenLDAP 2.5

Use gcc section attribute to keep less frequently used functions
away from main code.

Make sure the writer thread starts and stops when we expect it to.

And mdb_env_copyfd2(). Perform compaction on the copy. Trims out
freed pages and renumbers data pages in sequential order. This is
more CPU-intensive since it copies and modifies data pages.

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