Get rid of "args" global in V8
Attach V8Scope to v8::Isolate's Data

Frees the accessor data up for other uses
Cache conversion of string literals to V8 Strings
SERVER-6646 Fix more issues round-tripping V8 Strings with NUL bytes
Add V8String class to avoid copying v8::Strings into std::strings

Still need to copy once into v8::Utf8Value but this saves the second copy