fix(event_bus): panic if multiple goroutine call publish and then wait

[ken00535]

Because of wg's counter can't be a negative number, if call Wait() and then call Add(1) before Wait() complete, it will throw a panic "WaitGroup is reused before previous Wait has returned". so this PR add a mutex to fix the issue.

For more detail, see: https://stackoverflow.com/questions/48351816/waitgroup-is-reused-before-previous-wait-unknown-reason

I write a test TestSubscribeAsyncWithMultipleGoroutine that can reproduce the issue.

func TestSubscribeAsyncWithMultipleGoroutine(t *testing.T) {
	for i := 0; i < 100; i++ {
		bus := New()
		bus.SubscribeAsync("topic", func() {
			time.Sleep(time.Millisecond)
		}, false)
		bus.Publish("topic")
		go func() {
			time.Sleep(time.Millisecond)
			bus.Publish("topic")
		}()
		bus.WaitAsync()
	}
}

[codecov-commenter]

Codecov Report

Merging #49 (0c59636) into master (49d4230) will decrease coverage by 1.40%. The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master      #49      +/-   ##
==========================================
- Coverage   83.26%   81.86%   -1.41%     
==========================================
  Files           4        4              
  Lines         251      215      -36     
==========================================
- Hits          209      176      -33     
+ Misses         29       26       -3     
  Partials       13       13              

Impacted Files	Coverage Δ
event_bus.go	93.75% <100.00%> (-1.05%)	:arrow_down:
network_bus.go	76.66% <0.00%> (-2.75%)	:arrow_down:
server.go	72.13% <0.00%> (-1.79%)	:arrow_down:
client.go	77.27% <0.00%> (+0.34%)	:arrow_up:

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