Compare complex flow perf to imperative logic

 - Improve test dsl to better handle more event transformations

 - Rename "State" to "Event" for clarity

 - Allow for testing on more than 1 bg thread.

 - Add new DirectEventBuilder type

 - Break event-factory type into multiple interfaces so that
   benchmarks can be written to test functionalities on only
   a subset of event types if necessary.

Test: atest structured-concurrency-perf
Test: atest structured-concurrency-debug
Bug: 404377320
Flag: EXEMPT test
Change-Id: I0b1431dc9a062935d5344c3d0e5d2aea82b7d356