Fix some compile errors on node 0.10

* StackTrace::New doesn't exist
* Can't use NanReturnValue since it references "scope", which is
actually defined in Wrapper::Fun
Remove defunct Throw*Error helpers

Also remove some bogus assert() calls in async callback code
Better debug info when free() called incorrectly

Rewrite stmt.cancel() test
Add test for stmt.prepare() then stmt.execute() twice
Add TryCatch around and Unref() after threadpool callback
Implement S1118 handling for statement operations (though it shouldn't
be needed) and fix bug in S1118 handling for connections