http: allow async createConnection()

This commit adds support for async createConnection()
implementations and is still backwards compatible with
synchronous createConnection() implementations.

This commit also makes the http client more friendly with
generic stream objects produced by createConnection() by
checking stream.writable instead of stream.destroyed as the
latter is currently a net.Socket-ism and not set by the core
stream implementations.

PR-URL: https://github.com/nodejs/node/pull/4638


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: James M Snell <jasnell@gmail.com>