sys: process: add kill() and this_process::sleep() family

Add kill() member function to sys::process.
This function kills a process which the instance of sys::process has.
Add sys::this_process::sleep_for() and sleep_until()
The functionality is same as std::this_thread::sleep_for and sleep_until()
Make default ctor `default` (no explicit implementation)
Use new sys::this_process::sleep_for() in example/job_executor
README: remove wrong description