We actually compare the new key against the last key, to prevent
corruption.

The concurrency restriction is on threads within a single process.
Multiple processes can use mdb_dbi_open without affecting each other.

Revert "ITS#7789 Fix resize vs MDB_NOMETASYNC, and a comment."
Revert "ITS#7789 more"
Revert "Revert "ITS#7789 update RESIZED errmsg text""
Revert "ITS#7789 update RESIZED errmsg text"
Revert "ITS#7789 persist mapsize changes"

This reverts commit 02285aca.
This reverts commit 38e71c68.
This reverts commit a5e4eecb.
This reverts commit d8e18551.
This reverts commit 5926e54b.

Use DBI sequence numbers to detect DBIs being closed while in use.

Write decreases too, not just increases. Check for any size change
that was not requested by this process.

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.

Rename MDB -> LMDB
Integrate tool manpages

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.

Replace env flag MDB_LIVE_READER with field me_live_reader.

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.

It was inaccurate and misplaced. Verify it in mdb.c instead.

It and the MDB_msg_func can now return >= 0 for success.
Always return any MDB_msg_func() error result.

Handle keys stored by a liblmdb with a bigger MDB_MAXKEYSIZE.
mdb_get/mdb_del(absent key bigger than our MDB_MAXKEYSIZE)
now return MDB_NOTFOUND instead of MDB_BAD_VALSIZE.

Measured perf cost of meminit was 7% worst case on an Intel Core2
system. YMMV.