Implement SDL-0187 Restructure iOS Threading and Queueing

[joeljfischer]

Fixes #1028

This PR is ready for review.

Risk

This PR makes no API changes.

Testing Plan

Unit tests updated and smoke tests added

Summary

This PR restructures the threading in the library to use an "execution context" structure.

Changelog

Enhancements

• Restructure the threading in the library to use an "execution context" structure.

Tasks Remaining:

• Restructure SDLFileManager tests

CLA

• I have signed the CLA

[NicoleYarroch]

Left some comments.

[NicoleYarroch]

SYNC 3 doesn't seem to be handling multiple Show requests very well. Core either timeout after 20 seconds or response with a GENERIC_ERROR or INVALID_DATA request.

[joeljfischer]

After investigation, it seems that this is primarily a Ford issue with their HMI's handling of Show requests. Due to how the ScreenManager's internal TextAndGraphic and SoftButton managers are structured, and how the example apps work, there are multiple show requests happening immediately when certain soft buttons are pressed, and pressing them over and over very quickly leads to errors from Ford's head unit.

I was unable to determine that this happens at a more frequent rate than the previous 6.3.1 release, and testing this branch with a full-featured internal media app showed qualitative improvement in speed when connected to Ford's head unit.

Additional improvements will likely only be possible by batching together soft button and graphic Show updates. This isn't currently possible with the structure of the ScreenManager's internal managers, however, it may be possible once the TextAndGraphic sub-manager is restructured to use queues. This alone would be an improvement, and we could do further improvement by making the SoftButtonManager and TextAndGraphic queues combine updates when possible.

This would be quite a large bit of work, however, and is out of the scope of this proposal implementation, so I will leave that to future work.