benchmark: various tweaks for threads, randomness, vebosity etc

Several tweaks, nothing here affects production systems.

Stagger thread startup by 3 seconds to avoid swamping low-powered desktops
implement verbosity option for seleniumstudent benchmark
Use a thread-safe random() object to guarantee repeatability
Add extra waitfor events when creating students
Report the total number of exercise save attempts
Clear answer input box everytime, to avoid overflows
Fix next activity routing issue