The 'Child' framework used to perform the transfer of session quota
using 'env()->ram_session()' as hard-wired reference account. When
locally virtualizing the RAM session supplied to the 'Child', this
policy does not work. When closing a session, core would try to transfer
session quota to the virtualized RAM service, which is of course not
possible. This patch makes the reference RAM session configable via the
'Child_policy' interface.
