Collections API: Improve ExecutionContext Use

[rajadain]

Following the Future concurrency work done in #70, investigate the usage of a global ExecutionContext and provide alternatives if beneficial. Raster Foundry recently went through a similar phase of discovery and improvement. It would be nice if we could benefit from their experience.

For details, see @lossyrob's comment here: #70 (comment)

After talking with some of the GT team, I need to learn a bit more about how to properly use Execution Contexts in this setting (based on some painful work on the Raster Foundry team around their tiler). I'll give a report here when I sort things out.