piupartslib/packagesdb.py - Add rdep chain length metric calc.

Provide a means for detecting packages which support a long chain
of reverse dependencies, so that testing priorities can concentrate
on whittling them down.
piupartslib/packages.db - Prioritize by rdep chain len in reserve()

Use the rdep_chain_len metric to proportionally prioritize packages
returned by reserve() according to the length of the longest reverse
dependency chain length.