This simplifies the memory management of ContextifyContext,
making all references visible to V8.

The destructors don't need to do anything because when the wrapper is
going away, the context is already going away or otherwise it would've
been holding the wrapper alive, so there's no need to reset the
pointers in the context. Also, any global handles to the context
would've been empty at this point, and the per-Environment context
tracking code is capable of dealing with empty handles from contexts
purged elsewhere.

To this end, the context tracking code also purges empty handles
from the list now, to prevent keeping too many empty handles around.