Make Boost uFcontext the default ULT implementation on supported platforms
Boost uFcontext threads have lower context switching than any existing implementation we have, so should be made the default wherever they work. Separate issues should be opened for platforms where they don't work.
#4 Updated by Seonmyeong Bak about 1 month ago
Thread_local variables are not guaranteed to work with boost-context.
I think it works on linux but didn't work on OSX. So, if we want to make this as default ULT implementation then we should turn off thread local based `Cpv` implementation and use `Cpv` based on pthread_set/getspecific