librdkafka 1.3.0

Update librdkafka to 1.3.0, and miscellaneous packaging improvements.

Before merging this branch, please ensure that the repository is updated with the latest upstream.

# uscan --download doesn't work right now because the Github
# tags first page has too many -PRE* and -RC* tarballs
wget -O ../librdkafka_1.3.0.orig.tar.gz https://github.com/edenhill/librdkafka/archive/v1.3.0.tar.gz
gbp import-orig ../librdkafka_1.3.0.orig.tar.gz
gbp push