les: UDP pre-negotiation of available server capacity (#22183)

This PR implements the first one of the "lespay" UDP queries which
is already useful in itself: the capacity query. The server pool is making
use of this query by doing a cheap UDP query to determine whether it is
worth starting the more expensive TCP connection process.

common/prque/lazyqueue_test.go

@@ -40,7 +40,7 @@ type lazyItem struct {
 	index   int
 }
 
-func testPriority(a interface{}, now mclock.AbsTime) int64 {
+func testPriority(a interface{}) int64 {
 	return a.(*lazyItem).p
 }