src: fix Environment::GetCurrent() usage

Create a HandleScope before calling the Environment::GetCurrent() that
takes a v8::Isolate* as an argument because it creates a handle with
the call to v8::Isolate::CurrentContext().
src: remove global HandleScope

Make it more difficult to accidentally leak handles by removing the
top-level HandleScope.  Now if there's no valid HandleScope now, V8
will complain and, in debug builds, abort.
crypto: fix up implicit HandleScope abuse

Don't depend on any HandleScope objects that may or may not have been
created in src/stream_wrap.cc, create them explicitly.
src: fix Context::Scope usage

env->context() may or may not create a new Local.  It currently does
not but don't depend on that behavior, create a HandleScope first.
src: use Context::Scope objects in cares_wrap.cc

Enter the context explicitly, don't rely on the fact that there is a
Context::Scope a few stack frames below because it may be gone someday