softclock: Use dedicated ithreads for running callouts.

Rather than using the swi infrastructure, rewrite softclock() as a
thread loop (softclock_thread()) and use it as the main routine of the
softclock threads. The threads use the CC_LOCK as the thread lock
when idle.

Reviewed by: mav, imp, kib
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D33683