Fix actor race condition

* Use the `ConditionVariable` primitive to fix rece between stopped
  receiver Thread and its wake signal.
* Race was only observable via synthetical benchmarks and explicit
  sleeps between unlocking receiver mutex and Thread.stop
* Simplifies code a lot as thread must not be passed around anymore
Fix parenthesis
Add Subject#source_lines
Add Mutant::LineTrace

* Depends on `TracePoint` thus removes 1.9 compatiblity.
  I do not expect someone is harmed by this.
Add type field to jobs