[kweronsx]

This change:

• makes graphs retain events that are created during recording, so that destructors of event handles that release do not actually put the events back into the pool, from which they were allocated, as long as the graph, with which they are associated, is not destroyed.
• modifies the event pool allocate() function logic, so that events are not reused if they are still in use.
• adds a new flag that forces the even pool to create a new event, even if an event fit for reuse is available.
• adds tests that check if event allocation using the new logic works correctly.