xlog: use ev_sleep instead of fiber_sleep for rate limiting

fiber_sleep() works only if the current thread was created with
cord_costart(). Since vinyl worker threads don't need fibers, they
are created with cord_start() and hence can't use fiber_sleep().
So to be able to limit rate of vinyl dump/compaction, we have to
use ev_sleep() instead of fiber_sleep() in xlog. This is fine by
other xlog writers, because they don't use fibers either, neither
they should as xlogs are written without coio.

Needed for #3220