Refactor IAllocator code

The IAllocator class was handling all the requests on its own, passing
in parameters on top level which works, but is hard to maintain and not
flexible.

With the upcoming change to the IAllocator for MultiAllocate we can't
use the toplevel parameters anymore. Therefore, we refactor the code
into separate classes, which just do their own stuff. It works similiar
to Opcode containers.

Signed-off-by: René Nussbaumer <rn@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>