Add warning comment to getinfo

Warn that people should not add new information, or change current
information returned by getinfo.
Adds publishing blocks and transactions over ZMQ

ZMQ-based features are encapsulated in zmqports.{cpp,h} and disabled
if libzmq-dev is not found, or configure is invoked with
--disable-zmq.

Use -zmqpub=<endpoint> to use feature.

See contrib/zmq/README.md for details.