Code cleanups.
Removed unused parameters, and renamed property "executeOnFillable"
to "dispatchIO", for similarity with HttpClient and for better
description (it names the concept, not the implementation detail).
417356 - Add SOCKS support to jetty client.

Big refactoring to allow for additional proxy schemes that work at a
lower level than HTTP.

Introduced client-side ConnectionFactory, and binding that to a
HttpDestination, so that connections to that destination will use the
same ConnectionFactory.

The destination's ConnectionFactory is now initialized from the proxy
configuration and the transport, which is now itself a
ConnectionFactory.

The proxy configuration has also changed becoming polymorphic by
introducing a new ProxyConfiguration.Proxy abstract class,
which is implemented as HTTPProxy and can be implemented in future as
SOCKS4Proxy (and possibly others).
Merged branch 'jetty-9.1' into 'http_client_connection_factories'.