Split keepalive from pipelining in HTTP connections (bug https://bugs.ec...

...lipse.org/bugs/show_bug.cgi?id=434402). A new property 'pipelining' is added to HttpClient to disable HTTP pipelining (by default, pipelining is true to maintain backwards compatibility)
Tim, please see https://github.com/eclipse/vert.x/pull/876 for a solution to some of the issues with server side closed connections, the Connection: close header and pipelining.  It doesn't do the resend of pipelined requests but it does provide the caller with more insight into the failure rather than simply getting a request timeout with no knowledge if it was because of a connection close or not.