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 - Weight pkgs to be tested by rdep chain len.

... multiplied by the existing weighting by recursive reverse
dependency count.
Add changelog entry for reverse dependency chain length metric.