ReactiveSocket d'tor can be called from signal handler of a stream.

[ghost]

DO NOT MERGE!

This is still a WIP, to do:

• write a test to verify it actually works
• provide stronger guarantees to DuplexConnection, so that it's easier to get right.

[ghost]

This fixes #34.

[ghost]

So far, I know that:

• it's safe, since we hold a strong reference to ConnectionAutomaton, i.e. we cannot possibly dereference dangling pointer/reference,
• ConnectionAutomaton doesn't hold strong references to AbstractStreamAutomata, so there is no leak -- I actually verified that d'tor of ConnectionAutomaton is invoked,
• as a bonus, ConnectionAutomaton::endStream is still idempotent, which simplifies AbstractStreamAutomata.

[ghost]

GOOD TO MERGE (after review)

@yschimke, take a look at the test, please.

[yschimke]

The test makes sense to me, and other changes all looked appropriate given the pointer changes.