improve performance and correctness

* properly handle rebalances
* simplify code
* return on NewBroker instance and not interface

Signed-off-by: Vasiliy Tolstov <v.tolstov@unistack.org>

event.go

@@ -0,0 +1,42 @@
+package kgo
+
+import (
+	"sync"
+
+	"go.unistack.org/micro/v3/broker"
+)
+
+type event struct {
+	topic string
+	err   error
+	sync.RWMutex
+	msg *broker.Message
+	ack bool
+}
+
+func (p *event) Topic() string {
+	return p.topic
+}
+
+func (p *event) Message() *broker.Message {
+	return p.msg
+}
+
+func (p *event) Ack() error {
+	p.ack = true
+	return nil
+}
+
+func (p *event) Error() error {
+	return p.err
+}
+
+func (p *event) SetError(err error) {
+	p.err = err
+}
+
+var eventPool = sync.Pool{
+	New: func() interface{} {
+		return &event{msg: &broker.Message{}}
+	},
+}