This splits the `_find_coordinator_id()` method (which is blocking) into
request generation / response parsing methods.

The public API does not change. However, this allows power users who are
willing to deal with risk of private methods changing under their feet
to decouple generating the message futures from processing their
responses. In other words, you can use these to fire a bunch of requests
at once and delay processing the responses until all requests are fired.

This is modeled on the work done in #1845.

Additionally, I removed the code that tried to leverage the error
checking from `cluster.add_group_coordinator()`. That code had changed
in #1822, removing most of the error checking... so it no longer adds
any value, but instead merely increases complexity and coupling.

Previously the `sasl_kerberos_domain_name` was missing from the Admin
client. It is already present in the Consumer/Producer, and in all three
cases gets transparently passed down to the client.

Updated to include SASL_PLAINTEXT and SASL_SSL as options for security_protocol.

This breaks some of the consumer operations into request generation /
response parsing methods.

The public API does not change. However, this allows power users who are
willing to deal with risk of private methods changing under their feet
to decouple generating the message futures from processing their
responses. In other words, you can use these to fire a bunch of request
at once and delay processing the responses until all requests are fired.

Use `futures` to parallelize calls to `_send_request_to_node()`

This allows queries that need to go to multiple brokers to be run in parallel.

`getattr(object, 'x', object.y)` will evaluate the default argument
`object.y` regardless of whether `'x'` exists.

For details see: https://stackoverflow.com/q/31443989/770425