fix(Scope): more clean up scope on $destroy to minimize leaks

Due to a known V8 memory leak[1] we need to perform extra cleanup to make it easier
for GC to collect this scope object.

V8 leaks are due to strong references from optimized code (fixed in M34) and inline
caches (fix in works). Inline caches are caches that the virtual machine builds on the
fly to speed up property access for javascript objects. These caches contain strong
references to objects so under certain conditions this can create a leak.

The reason why these leaks are extra bad for Scope instances is that scopes hold on
to ton of stuff, so when a single scope leaks, it makes a ton of other stuff leak.

This change removes references to objects that might be holding other big
objects. This means that even if the destroyed scope leaks, the child scopes
should not leak because we are not explicitly holding onto them.

Additionally in  theory we should also help make the current scope eligible for GC
by changing properties of the current Scope object.

I was able to manually verify that this fixes the problem for the following
example app: http://plnkr.co/edit/FrSw6SCEVODk02Ljo8se

Given the nature of the problem I'm not 100% sure that this will work around
the V8 problem in scenarios common for Angular apps, but I guess it's better
than nothing.

This is a second attempt to enhance the cleanup, the first one failed  and was
reverted because it was too aggressive and caused problems for existing apps.
See: #6897

[1] V8 bug: https://code.google.com/p/v8/issues/detail?id=2073

Closes #6794
Closes #6856