Fixes bug 7959; bugfix on 0.2.4.8-alpha.

Looks like Roger's debugging code wanted to take a tour of the world
outside his sandbox.

This reverts part of commit 19d37202.

Otherwise, it's possible to create streams or circuits with these
bogus IDs, leading to orphaned circuits or streams, or to ones that
can cause bandwidth DOS problems.

Fixes bug 7889; bugfix on all released Tors.

Fixes bug 7935.  Reported by 'oftc_must_be_destroyed'.

Also add in the random nonce generation.

In general, if we tried to use a circ for a stream, but then decided to place
that stream on a different circuit, we need to probe the original circuit
before deciding it was a "success".

We also need to do the same for cannibalized circuits that go unused.

Fix cannibalize, rend circ and intro circ timeout handling.

Makes bug 7869 more easily fixable if we ever choose to do so.

Conflicts:
	src/or/cpuworker.c
	src/or/or.h
	src/test/bench.c

This should make the intent more explicit.  Probably needless, though.

Before I started coding ntor in C, I did another one in Python.
Turns out, they interoperate just fine.