shorten the sleep in isc_nm_destroy()

shorten the sleep in isc_nm_destroy()

when isc_nm_destroy() is called, there's a loop that waits for
other references to be detached, pausing and unpausing the netmgr
to ensure that all the workers' events are run, followed by a
1-second sleep. this caused a delay on shutdown which will be
noticeable when netmgr is used in tools other than named itself,
so the delay has now been reduced to a hundredth of a second.

(cherry picked from commit 870204fe47b093b3a0ff43f6f20696c63b9582d5)