src: fix Environment::GetCurrent() resource leak

Environment::GetCurrent(Isolate*) makes a call to
Isolate::GetCurrentContext(), which returns a Local<Context>. Hence,
every call to GetCurrent(Isolate*) should be made within a HandleScope
or EscapableHandleScope.

Also removed excessive calls to GetCurrent().

/cc @bnoordhuis @indutny 
It would be much nicer actually, if v8 would accept a scope, as an argument for a `Local` constructor. This what it will be much more explicit and will be statically verifiable.