Add scheduler_active flag and PLS_SERIAL_ELUSION compile option.

The flag can be used to run code annotated with PLS outside of a scheduler environment,
i.e. the app does not crash if the code is called without context. The compile option
allows to omit all spawn and sync calls during compilation, creating the equivalent serial code.
3 jobs from master in 3 minutes 57 seconds (queued for 1 second)
Status Job ID Name Coverage
  Build
passed #3830
build_cmake

01:01

 
  Test
passed #3831
run_tests

00:58

 
  Sanitizer
passed #3832
run_address_sanitizer

01:58