Remove pluggable global connectors

We only have one connector (the system connector), so get rid of the abstraction.
Also, since the system needs to have specialized logic for dealing with sys tables,
get rid of the system connector. We'll revisit this once we figure out how we
want to model it going forward.
Don't forward requests for well-known schemas to connector

This requires hard-coding knowledge about sys and information_schema in
MetadataManager. It's a hack until we figure out how we want to model
these going forward.
Create AsyncResponseHandler builder pattern

- Replaces AsyncResponseUtils
- Adds ability to asynchronously cancel on client disconnect
Include response body in remote task error message
[cassandra] Bump to current stable version.

The 2.0.0-rc2 driver has a bug
(https://datastax-oss.atlassian.net/browse/JAVA-254) which gets
triggered when querying through presto.  An exception occurs because the
driver attempts to query a table that no longer exists, and fails.

Cassandra 2.1.0 is the current stable release, so it's probably time to
catch up.  The hector library has been removed and replaced, because it
didn't seem to work correctly with C* 2.1, and the project doesn't
appear to be very active.