test: don't call process.exit() in debugger tests

process.exit() tends to hide bugs, both in tests and node.js.
Rewrite the tests so that the event loop exits naturally.
src: add multi-context support

This commit makes it possible to use multiple V8 execution contexts
within a single event loop.  Put another way, handle and request wrap
objects now "remember" the context they belong to and switch back to
that context when the time comes to call into JS land.

This could have been done in a quick and hacky way by calling
v8::Object::GetCreationContext() on the wrap object right before
making a callback but that leaves a fairly wide margin for bugs.

Instead, we make the context explicit through a new Environment class
that encapsulates everything (or almost everything) that belongs to
the context.  Variables that used to be a static or a global are now
members of the aforementioned class.  An additional benefit is that
this approach should make it relatively straightforward to add full
isolate support in due course.

There is no JavaScript API yet but that will be added in the near
future.

This work was graciously sponsored by GitHub, Inc.
src: fix multi-base class ObjectWrap::Unwrap<T>()

Fix pointer unwrapping when T is a class with more than one base class.

Before this commit, the wrapped void* pointer was cast directly to T*
without going through ObjectWrap* first, possibly leading to a class
instance pointer that points to the wrong vtable.

This change required some cleanup in various files; some classes
used private rather than public inheritance, others didn't derive
from ObjectWrap at all...

Fixes #6188.
process: Add exitCode property

This allows one to set a specific status code, while still letting the
process exit gracefully once all async operations are completed.