stream: fix _final and 'prefinish' timing

[ronag]

This PR fixes a few different things:

The timing of 'prefinish' depends on whether or not
_final is defined. In one 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.

The stream should not finish if errored or destroyed
synchronously during the prefinish stage.

Refs: #31401
Refs: #32763 (comment)

Checklist

• make -j4 test (UNIX), or vcbuild test (Windows) passes
• tests and/or benchmarks are included
• documentation is changed or added
• commit message follows commit guidelines