Speed up twisted.protocols.tls shutdown by reducing number of roundtrips

[itamar]

t.p.tls shutdown currently works like this -

1. A to B: TLS shutdown, please.
2. B to A: OK, TLS shutdown. TCP FIN too.
3. A to B: TCP FIN.

Using half-close, this could be reduced to:

1. A to B: TLS shutdown, please. TCP FIN.
2. B to A: OK, TLS shutdown. TCP FIN too.

Attached is a patch (against #5118 branch) that implements a working sketch.

[itamar]

Patch against #5118 branch (or trunk, if merged)