SERVER-9538 Only update BufBuilder's len if successfully grew

Otherwise BSONObj destructor will try to grow again to add EOO byte,
causing a double exception and terminate() call.
Include What You Use for bufreader.h
SERVER-7172 Build support for creating a path in a mutable document.
SERVER-7172 Support testing whether a field is updatable.
SERVER-7172 Remove unused code in test fixtures.