src: fix handle leak in Parser::Execute()

Fix a resource leak where an intermediate Local<Context> handle in
Environment::GetCurrent() got leaked into whatever HandleScope was
further up the stack.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
src: add ClearWrap() to util.h

Counterpart to Wrap(), clears the previously assigned internal field.
Will be used in an upcoming commit.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
lib, src: don't make http parser handles weak

Weak handles put strain on the garbage collector and the parser handle
doesn't need to be weak in the first place.  This change should improve
GC times on busy servers a little.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
src: update and expand comments in base-object.h

It's safe to call BaseObject::object() from your destructor _unless_
the handle is weak; then it's the weak callback that is calling your
destructor and the object will have been released by the time the
destructor runs.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
lib: http: poison parser references after freeing

Make it a little harder to slip in use-after-free bugs by nulling out
references to the parser object after handing it off to freeParser().

Reviewed-by: Trevor Norris <trev.norris@gmail.com>
lib: unexport http.parsers

Unexport the http.parsers freelist.  It was originally exported by Ryan
in commit 0003c701 but the commit log doesn't mention why and it's never
been documented.  It's unclear if there are any users.

The lifecycle of parser objects changed recently and it seems better to
not let people shoot themselves in the foot so easily.

If it turns out there are actually users, we can always re-export it
again - probably under a slightly different name, to force people to
update their code to the new way of things.

Reviewed-by: Trevor Norris <trev.norris@gmail.com>