This commit adds support for queued scheduling with slot sharing to the
SlotPool. The idea of slot sharing is that multiple tasks can run in the
same slot. Moreover, queued scheduling means that a slot request must not
be completed right away but at a later point in time. This allows to
start new TaskExecutors in case that there are no more slots left.

The main component responsible for the management of shared slots is the
SlotSharingManager. The SlotSharingManager maintains internally a tree-like
structure which stores the SlotContext future of the underlying
AllocatedSlot. Whenever this future is completed potentially pending
LogicalSlot instantiations are executed and sent to the slot requester.

A shared slot is represented by a MultiTaskSlot which can harbour multiple
TaskSlots. A TaskSlot can either be a MultiTaskSlot or a SingleTaskSlot.

In order to represent co-location constraints, we first obtain a root
MultiTaskSlot and then allocate a nested MultiTaskSlot in which the
co-located tasks are allocated. The corresponding SlotRequestID is assigned
to the CoLocationConstraint in order to make the TaskSlot retrievable for
other tasks assigned to the same CoLocationConstraint.

Port SchedulerSlotSharingTest, SchedulerIsolatedTasksTest and
ScheduleWithCoLocationHintTest to run with SlotPool.

Restructure SlotPool components.

Add SlotSharingManagerTest, SlotPoolSlotSharingTest and
SlotPoolCoLocationTest.

This closes #5091.