Use the latest meta page

Allow both increasing and decreasing the environment size.
But don't allow decreasing below the currently occupied space.

These ops could fail if the map is full.

Don't try to reclaim overflow pages while operating on
the freelist (for now). The circular dependencies are much like
the single-page case, but worse. Maybe look into this in the
future, but it's not absolutely necessary now.

Keep list sorted if it grows during a write. Don't free pghead until
we're sure our writes are all finished.

Just write in 2^31 sized chunks

Avoid possible symbol clashes with MDB-Tools libmdb

mdb_cursor_sibling() no longer pops cursor before returning.

liblmdb