Summary:
If you use a null eventbase, folly implicitly decides which one to run the futures in, for example
the timekeeper thread.
The only component that passes in event bases is sonar.cpp.
Reviewed By: priteshrnandgaonkar
Differential Revision: D9539443
fbshipit-source-id: 7fd7289257c84b039a7ac00b14f78bb271262480