deadlock in Close

[ykalayy]

Problem

Whenever eviction and close run at the same time. There is a possible deadlock because of the closeLock mutex usage on poo.go:487. We can unlock it whenever we don't need it rather than using defer.

Repreduce UT

func (suit *PoolTestSuite) TestConcurrentCloseAndEvict() {
	ctx := context.Background()
	suit.pool.Config.MinIdle = 1
	suit.pool.Config.SoftMinEvictableIdleTime = time.Millisecond * 100
	suit.pool.Config.TimeBetweenEvictionRuns = time.Millisecond * 500
	suit.factory.destroyLatency = time.Millisecond * 1000 // Destroy takes 1000 ms
	suit.pool.PreparePool(ctx)
	suit.pool.StartEvictor()
	suit.Equal(1, suit.pool.GetNumIdle())
	ticker := time.NewTicker(time.Millisecond * 1000)
	testTimeoutTicker := time.NewTicker(time.Millisecond * 5000) // if time exceeds test fails
	go func() {
		select {
		case <-testTimeoutTicker.C:
			// Time-out
			suit.FailNow("Time is exceeds on pool close")
		}
	}()
	select {
	case <-ticker.C:
		suit.pool.Close(ctx)
	}
}

[ykalayy]

@jolestar Could you please review my PR?