Default value of MaxIdleConnections in net2.ConnectionPool is extremely slow

Benchmark results:

```
Fake dealer:
BenchmarkGetConnection-16 1492227 803.2 ns/op
BenchmarkGetConnectionMaxIdle-16 1888982 631.4 ns/op
Real dealer:
BenchmarkGetConnection-16 9052 113356 ns/op
BenchmarkGetConnectionMaxIdle-16 1901071 626.9 ns/op
```

Code example:

```
func BenchmarkGetConnection(b *testing.B) {
// dialer := fakeDialer{}
// mockClock := time2.MockClock{}

options := ConnectionOptions{
// MaxIdleConnections: 1,
// Dial: dialer.FakeDial,
// NowFunc: mockClock.Now,
}

pool := NewSimpleConnectionPool(options)
pool.Register("tcp", "localhost:11211")
b.ResetTimer();
for i := 0; i < b.N; i++ {
c, err := pool.Get("tcp", "localhost:6379")
if err != nil {
b.Fatal(err)
}
c.ReleaseConnection()
}
}

func BenchmarkGetConnectionMaxIdle(b *testing.B) {
// dialer := fakeDialer{}
// mockClock := time2.MockClock{}

options := ConnectionOptions{
MaxIdleConnections: 1,
// Dial: dialer.FakeDial,
// NowFunc: mockClock.Now,
}

pool := NewSimpleConnectionPool(options)
pool.Register("tcp", "localhost:11211")
b.ResetTimer();
for i := 0; i < b.N; i++ {
c, err := pool.Get("tcp", "localhost:6379")
if err != nil {
b.Fatal(err)
}
c.ReleaseConnection()
}
}
```