raft: Avoid scanning raft log in becomeLeader

Scanning the uncommitted portion of the raft log to determine whether
there are any pending config changes can be expensive. In
cockroachdb/cockroach#18601, we've seen that a new leader can spend so
much time scanning its log post-election that it fails to send
its first heartbeats in time to prevent a second election from
starting immediately.

Instead of tracking whether a pending config change exists with a
boolean, this commit tracks the latest log index at which a pending
config change *could* exist. This is a less expensive solution to
the problem, and the impact of false positives should be minimal since
a newly-elected leader should be able to quickly commit the tail of
its log.

raft/node_test.go

@@ -348,6 +348,7 @@ func TestNodeProposeAddDuplicateNode(t *testing.T) {
 				n.Tick()
 			case rd := <-n.Ready():
 				s.Append(rd.Entries)
+				applied := false
 				for _, e := range rd.Entries {
 					rdyEntries = append(rdyEntries, e)
 					switch e.Type {
@@ -356,10 +357,13 @@ func TestNodeProposeAddDuplicateNode(t *testing.T) {
 						var cc raftpb.ConfChange
 						cc.Unmarshal(e.Data)
 						n.ApplyConfChange(cc)
-						applyConfChan <- struct{}{}
+						applied = true
 					}
 				}
 				n.Advance()
+				if applied {
+					applyConfChan <- struct{}{}
+				}
 			}
 		}
 	}()