device: combine debug and info log levels into 'verbose'

There are very few cases, if any, in which a user only wants one of
these levels, so combine it into a single level.

While we're at it, reduce indirection on the loggers by using an empty
function rather than a nil function pointer. It's not like we have
retpolines anyway, and we were always calling through a function with a
branch prior, so this seems like a net gain.

Signed-off-by: Jason A. Donenfeld <Jason@zx2c4.com>

device/peer.go

@@ -190,7 +190,7 @@ func (peer *Peer) Start() {
 	}
 
 	device := peer.device
-	device.debugf("%v - Starting...", peer)
+	device.log.Verbosef("%v - Starting...", peer)
 
 	// reset routine state
 
@@ -278,7 +278,7 @@ func (peer *Peer) Stop() {
 	peer.routines.Lock()
 	defer peer.routines.Unlock()
 
-	peer.device.debugf("%v - Stopping...", peer)
+	peer.device.log.Verbosef("%v - Stopping...", peer)
 
 	peer.timersStop()