973784 - improving performance of depsolve

Using the pre-calculated serialized version and release as stored in the database to do version comparison rather than re-calculating on the fly. This along with a couple of other tweaks has resulted in a 40x speedup.

https://bugzilla.redhat.com/show_bug.cgi?id=973784
``tuple`` is not the correct type for that data structure. If found, it would be a ``Unit`` instance.