[OpenMP][CUDA] Use one event pool per device

An event pool, similar to the stream pool, needs to be kept per device.
For one, events are associated with cuda contexts which means we cannot
destroy the former after the latter. Also, CUDA documentation states
streams and events need to be associated with the same context, which
we did not ensure at all.

Differential Revision: https://reviews.llvm.org/D120142