Use std::lock_guard instead of mutex lock for FSR generators

Summary:
Use std::lock_guard instead of mutex lock for FSR generators.

This is related with !30 and T168.

Test Plan: Tests give identical results with respect to master. CPU time remains the same.

Reviewers: kreps, tlatham, jback

Reviewed By: tlatham

Tags: #evtgen

Differential Revision: https://phab.hepforge.org/D127