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

Avoid naming conflicts with other mdb* packages. Bump version to 0.9.5