Add support to cancel SDK network requests

[bmilekic]

Consider the following scenario:

• targeting() being called by app developer, to update local targeting data cache every time the app is foregrounded
• user rapidly backgrounds-foregrounds the app
• currently, this results in several network requests being sent to backend

A mitigation could be that any pending targeting request is cancelled prior to a new one being dispatched.

[eugenedorfman-optable]

The uses of targeting() API call can be diverse - not only on app foregrounding, thus to guarantee completion / cancellation of the prior request - we would need to implement a serial queue of targeting request "jobs".

A typical scenario is prior to a Prebid/ad request - the client checks targetingFromCache - if targeting data is unavailable (subject to certain TTL) - call targeting (that sends a request to the Edge API internally and stores the result in cache for the future and immediate use). Otherwise use data from cache. In this scenario we don't expect a lot of parallel requests to targeting API. Potentially parallel requests could happen if there is more than one ad unit per screen and each wants to enrich its own Prebid request (unlike Prebid.js - Prebid Mobile SDK has single ad unit auctions), however as soon as data is cached this request flood should stop.

Maintaining a serial targeting requests queue inside Optable SDK is feasible, but might be an overkill.