[C++] ReadAt/WriteAt are inconsistent with moving the files position

Right now, there is inconsistent behaviour regarding moving the files position pointer after calling ReadAt or WriteAt. For example, the default implementation of ReadAt seeks to the desired offset and calls Read which moves the position pointer. MemoryMappedFile::ReadAt, however, doesn't change the position. WriteableFile::WriteAt seem to move the position in the current implementation, but there is no docstring which prescribes this behaviour.

Antoine suggested that *At methods shouldn't touch the position and it makes more sense, IMHO. The change isn't huge and doesn't seem to break anything internally, but it might break the existing user code.