pkg/log: add Clear to stop debug logging (#3372)

When reloading we need to disable debug output when the debug plugin is
removed from the config file. Add a `Clear` function to pkg/log and use
it in the server server.

Add test case in pkg/log, for actuall check I manually checked the
output by sprinkling some debug statements in the startup and checking
with sending SIGUSR1.

Also clear up the comments in pkg/log to remove the text about time
stamping.

Fixes: #3035

Signed-off-by: Miek Gieben <miek@miek.nl>

core/dnsserver/server.go

@@ -64,6 +64,10 @@ func NewServer(addr string, group []*Config) (*Server, error) {
 		if site.Debug {
 			s.debug = true
 			log.D.Set()
+		} else {
+			// When reloading we need to explicitly disable debug logging if it is now disabled.
+			s.debug = false
+			log.D.Clear()
 		}
 		// set the config per zone
 		s.zones[site.Zone] = site