[SDL 0087] Sequentially Send Multiple RPCs

[theresalech]

Proposal: Sequentially Send Multiple RPCs

This proposes to add an API allowing the developer to sequentially send multiple RPCs and to be notified when all have completed sending.

Review: smartdevicelink/sdl_evolution#263

Steering Committee Decision:

The Steering Committee voted to accept this proposal with the following revisions: revise to add Android counterpart to this proposal. Additionally, when implementing, will need to investigate at what rate RPCs can be sent and received, as well as which RPCs do not need to be sent asynchronously. Revisions to add the Android counterpart are now included within the .md file.