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
For a recursive copy from a repo with over 12k rpms (rhel6.5), this change reduced the total time by about 1.3 seconds, out of a total time of about 28 seconds.