node: change AsyncListener API

There was a flaw in the old API that has been fixed. Now the
asyncListener callback is now the "create" object property in the
callback object, and is optional.
node: properly check uid when adding AsyncListener

Instead of checking the uid on the array index of the queue, instead the
object property "uid" was checked on the queue iteself. Because this
will always evaluate to "undefined" the same listener could be added
multiple times to the same context.
crypto: introduce .setEngine(engine, [flags])
doc: Add forward secrecy section to TLS docs

This fixes confusion connected to comparison of ECDH
with RSA and wrong information on forward secrecy.