Refactor handling of response's 'end' in forwarder

[DamienCassou]

The only purpose of this refactoring is to simplify the implementation
of a new feature.

This refactoring propagates the limitation that the error code is a
string of only one character. This limitation is already present in
the code handling the 'readable' event.

It would be better to remove this limitation though but I wanted your
point of view on this first as it requires some more refactoring.

[mantoni]

Yes, I agree that it would be better to remove the limitation 👍

[DamienCassou]

I have started refactoring. Feel free to give early feedback if you want to.

[DamienCassou]

@mantoni This PR is finally ready.

The fact that the last commit has to change all these tests is a smell in my opinion. I believe these unit tests should test more the behavior of the application and less the implementation details. This is out of scope for this PR, but I would replace code like this:

assert.calledWith(process.stdout.write, 'res');
assert.calledWith(process.stdout.write, 'ponse from ');
assert.calledWith(process.stdout.write, 'daemon');

with something like:

assertWritten(process.stdout, 'response from daemon');

assertWritten(stream, expectedMessage) would be a function that concatenates the first argument of every call to stream.write() and compare that against expectedMessage.

[mantoni]

Thank you for kicking this off. I merged #306 which includes your work.