Create simulator interface which provides thread scheduling + speculative fetching

Rather than having each simulator figure out how to schedule traced software threads onto simulated cores in their own ad hoc way, we would like to provide a scheduler service, which should result in several benefits:

• Ease of use: a new simulator use case has one less thing to implement
• Consistency: all simulators can now use the same approach
• Fill in gaps in trace-based simulation:
  • We can re-schedule threads even when simulating the recorded hardware to deflate context switches increased by tracing overhead
  • We can more easily combine multiple single-workload traces
  • We can provide speculative path fetching using various schemes (from heuristics to additional data recorded during tracing) to help bridge the gap with execution-driven simulation

Xref #5694: provide per-core iterator. That may become subsumed by this new broader-scope feature.