Improved Prefetching Support

[domoritz]

Currently the Table component and the RasterTile mark leverage the Coordinator’s prefetch method to speculatively issue queries ahead of time and cache the results for future use. One next step is to explore ways for more automatic prefetching support within vgplot marks. For example, when diving into a time-series or density-based plots, might the system automatically determine a tiling scheme, perform prefetching for adjacent tiles, and stitch data tiles together as needed to meet current data needs? What reusable abstractions might we develop and deploy? A stretch goal would be to also develop more sophisticated prefetching strategies beyond requesting adjacent tiles, perhaps inspired by predictive approaches such as ForeCache.