Asynchronous call for the dispatcher to avoid blocking request when doing decisions

[mdprotacio]

Summary

• Dispatcher is supposed to be for asynchronous operation that can be done outside of the normal process like the sending of impression, however, the call to the sending of impression is synchronous and is thus creates a bottleneck in terms of performance. By making the call asynchronous, it improves the performance of decide call.

[mikechu-optimizely]

@mdprotacio Thanks for opening a PR. We'll take a look at your suggestion shortly.

[mdprotacio]

As issues can be addressed by implementing our own dispatcher, I'm closing this PR.

[mikechu-optimizely]

Thanks for the update.