fix: use proto.Equal to compare protobuf messages (#125)

replace reflect deepequal with proto.Equal

reflection is comparing some internal fields used during marshaling, causing race
conditions in the beats reloader code.
the race condition causes APM Server tests to fail with the race detector enabled

pkg/client/unit.go

@@ -5,7 +5,6 @@
 package client
 
 import (
-	"reflect"
 	"sync"
 
 	gproto "google.golang.org/protobuf/proto"
@@ -206,7 +205,7 @@ func (u *Unit) UpdateState(state UnitState, message string, payload map[string]i
 	}
 	if (u.statePayload == nil && statePayload != nil) ||
 		(u.statePayload != nil && statePayload == nil) ||
-		!reflect.DeepEqual(u.statePayload, statePayload) {
+		!gproto.Equal(u.statePayload, statePayload) {
 		u.statePayload = statePayload
 		changed = true
 	}