Metadata: reread client list from database

This fixes two related bugs: One causes Metadata to use an out-of-date
cached list of clients when a client is deleted or added with
bcfg2-admin; the other causes child worker processes to use an
out-of-date cached list of clients when a client is added with a Bcfg2
run when the multiprocessing core is in use.
bcfg2-lint: Verify abstract Package tags
helpers: better error message when failing to read FileBacked
redhat: fixed killproc behavior when multiprocessing core doesn't shut down properly