Enhancement request: `.close()` should wait for in-flight requests to complete

[clementallen]

How would the enhancement work?

Currently, the optimizely.close() Promise will send and wait for any pending events. However, it does not wait for the completion of events that are in the progress of being sent.

When would the enhancement be useful?

To avoid events half way through being sent getting cancelled before sending is complete.

@optimizely/optimizely-sdk version: 3.3.2

[mjc1283]

Thanks @clementallen for filing this issue. As mentioned in #397, this functionality is available in v3.5.0. Let us know if you have any further comments or questions.