Maximum number of events in an array

Currently the `add_event()` method in `cl.array` has a hard coded limit of 12 for the number of queued events. I think it would be better if this was a configurable property. I'm trying to use gpyfft while using the build-in event management in `cl.array`. However, gpyfft has a hard-coded limit `DEF MAX_WAITFOR_EVENTS = 10` which is lower than in `add_event()` (and leads to an assertion error), basically forcing me to periodically call `cl.array.finish()` which is nonideal. I would like to be able to do
```
arr.set_max_events(10)
```