Require explicit general database closing.

To be idiomatic and play nicely as a library, control of the database
should exist solely in the hands of the library user and not in that
of the memory manager.

Major Caveat: A number of internal types around iterators and versions
retain their finalizers for now.  I need to take some time to diagram
their lifecycle to propose a solution for this.  This domain is where,
I believe, the nondeterminism I found was, but I want to start fixing
the easy part first.

@syndtr, if you have some recommendations for
how to approach this, I would love to talk with you.  If you look at other
LevelDB implementations, they explicitly require the user to call
``Iterator.Close`` to free associated resources, including the an
official but incomplete implementation from Google in Go at
https://code.google.com/p/leveldb-go/source/browse/leveldb/db/db.go#72.