rdsamsa: metadata adapters mostly complete
rdsamsa: metadata adapters mostly complete

This implements most of abstract.Cluster/Broker/Topic/Partition
(although latest/earliest_offsets are still missing as they will have to
be taken from stats_cb in librdkafka - soon in another commit).

I've informally tested Cluster et al; they're accessible and updateable
in an interactive session in a local python-librdkafka development
container, but obviously I'll have to commit proper tests.  These will
follow asap, after we've first made it easy/automatic to install
python-librdkafka in a samsa dev-environment.

Signed-off-by: Yung-Chin Oei <e4c1ab9da74060df152545062ee274845648ed6c@yungchin.nl>

vortec/pykafka,jofusa/pykafka,aeroevan/pykafka,wikimedia/operations-debs-python-pykafka,vortec/pykafka,benauthor/pykafka,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,fortime/pykafka,sontek/pykafka,fortime/pykafka,sontek/pykafka,sammerry/pykafka,thedrow/samsa,appsoma/pykafka,tempbottle/pykafka,aeroevan/pykafka,wikimedia/operations-debs-python-pykafka,benauthor/pykafka,jofusa/pykafka,thedrow/samsa,yungchin/pykafka,tempbottle/pykafka,thedrow/samsa,sammerry/pykafka,yungchin/pykafka,appsoma/pykafka