dcs-package-importer: skip full.idx when merging

Otherwise the new index will include the old one and quickly grow over
the size limit of 4GB.
add dcs-tail-fedmsg
dcs-common: set GOMAXPROCS=4 by default

The virtual machines which we are going to use have 4 cores.
dcs-source-backend: fix path to binary
add dcs-feeder, feeds packages to dcs-package-importer instances

This is either done periodically (every hour) by comparing the Sources
files from Debian mirrors with what’s present on the
dcs-package-importer shards, or on demand when dcs-tail-fedmsg sends a
/lookfor request.
add dcs-index-backend.service