Uses WeakSet to store events so we don't have to worry when they get
destroyed.

The check for out of sync events is also more strict: we now store the
event per (buffer, op) as well as per queue and only check events that
are still valid in both and only for the current buffer.

Added the following logic:
* in `enqueue_nd_range_kernel`, we catch all generated events for each
queue.
* those events get cleared when calling `queue.finish` or
`cl.wait_for_events`.
* if a kernel argument's queue's events are not in the current `wait_for`,
we complain.

Implement get_async

Closes #18

See merge request inducer/pyopencl!91

docs(README.rst): embrace https links

Changed links that were in http protocol and confirmed supporting https to https.

Changed links that were in http protocol and confirmed supporting https to https.

Do not release gil in nanny event destructor

See merge request !92

Make Conda Apple CI use Conda-installed Khronos ICD loader and pocl

See merge request !90