Add support for using an rpmdb on a NoSQL! distributed store

Using mongodb to store the state remotely in a
distributed NoSQL! store is the easiest path to
install state being saved remotely rather than locally.

The mongo-c-driver is already bundled "batteries included" in rpm-5.3.x.

A per-client host identifier scheme needs to be devised so that
multiple rpmdb's can be co-resident in a distributed store.

The basic methods needed are
    open/close/get/put/del
with a little though about saving active query state in the
equivalent of a bdb cursor.

A secondary -> primary lookup will need to be devised. The
primary store of header blobs may need a look-aside cache
eventually (which Berkeley DB could be ironically useful).