Move recording code from recorder.cc to RecordSession and put it behind a record_step() API.
Remove ensure_preload_library_can_load and move other environment setup code to RecordSession.

Failure to load the preload library is not very common and checking it does
an extra fork/exec of rr for every rr recording. If we still need this, we
should figure out a more efficient way to do the check.
Remove unneeded #includes.
Rename recorder_sched.cc/h to Scheduler.cc/h and move logic and data into a Scheduler object.