Remove redundant code in IAllocator class

This moves the setting of the request member on the in_data, of the
request type, and of the branching basef on request type outside of
individual functions and directly into the constructor.

Since the values we're using externally are identical to the
constants.py values, we're also using those directly.

Signed-off-by: Iustin Pop <iustin@google.com>
Reviewed-by: Michael Hanselmann <hansmi@google.com>