stream: fix _final and 'prefinish' timing

This PR fixes a few different things:

The timing of 'prefinish' depends on whether or not
_final is defined. In on case the event is emitted
synchronously with end() and otherwise asynchronously.

_final is currently unecessarily called asynchronously
which forces implementors to use 'prefinish' as a hack
to emulate synchronous behaviour. Furthermore, this hack
is subtly broken due to the above issue.

Refs: https://github.com/nodejs/node/issues/31401
Refs: https://github.com/nodejs/node/pull/32763#discussion_r407041983

PR-URL: https://github.com/nodejs/node/pull/32780


Reviewed-By: Matteo Collina <matteo.collina@gmail.com>
Reviewed-By: Rich Trott <rtrott@gmail.com>