Shared: Make UniversalFlow overlay-aware.

[aschackmull]

This updates the UniversalFlow library to be overlay-aware. Since the output is semantically local, the library can restrict the global computation to target the overlay and thereby become much more incremental in nature. The local output is tagged overlay[local?] in order to also support C++, for which the node type is global and hence cannot currently use incremental computation. So only Java gets the incremental computation for now.