Factored out bits common to resource caches, added global limit on number of
resources being requested at any one time.
Merge branch 'master' of https://github.com/highfidelity/hifi
Further work on request limiting.
Merge pull request #2136 from ey6es/master

Factored out common resource cache functionality, added limits to simultaneous downloading (prioritized by distance).  Closes #2122.