Add full support for `use_parent`.

If specified, the input files passed to the job are not a flat list of
input files anymore, but a list of hashes that the parameter set tweaking
of WMCore expects.  The latter then takes care of setting up CMSSW properly
for us.

Fixes #4514.
Create database entry for `use_parent` also for MySQL.
Raise an exception when a file has no parents, but parent usage is requested.
Merge pull request #4550 from matz-e/use_parent

Complete support for `use_parent`.