stream_base: dispatch reqs in the stream impl

Dispatch requests in the implementation of the stream, not in the code
creating these requests. The requests might be piled up and invoked
internally in the implementation, so it should know better when it is
the time to dispatch them.

In fact, TLS was doing exactly this thing which led us to...

Fix: https://github.com/iojs/io.js/issues/1512
PR-URL: https://github.com/iojs/io.js/pull/1563


Reviewed-By: Shigeki Ohtsu <ohtsu@iij.ad.jp>