Avoid signals for JS interruptability

SpiderMonkey offers support for non-signal based interruption via JS_SetGlobalJitCompilerOption in JSJITCOMPILER_SIGNALS_ENABLE mode.

This appears to solve the need for called code to guard against signals, as well as the general system instability caused by using signals (variables swapping values in js, other code crashing, etc).

Switch to invoking those methods at runtime setup and re-enable periodic interruption