When connection pool is organized as a stack,
it always returns most recently used connection.
Because each connection has its own page cache,
using the connection with fresh cache improves performance.
I commented out `oauth2::tests::test_oauth_from_mx`
because it requires network connection,
turned off the Wi-Fi and ran the tests.
Before the change, with a queue:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
Time (mean ± σ): 56.424 s ± 4.515 s [User: 183.181 s, System: 128.156 s]
Range (min … max): 52.123 s … 68.193 s 10 runs
```
With a stack:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
Time (mean ± σ): 29.887 s ± 1.377 s [User: 101.226 s, System: 45.573 s]
Range (min … max): 26.591 s … 31.010 s 10 runs
```
On version 1.107.1:
```
$ hyperfine "cargo test"
Benchmark 1: cargo test
Time (mean ± σ): 43.658 s ± 1.079 s [User: 202.582 s, System: 50.723 s]
Range (min … max): 41.531 s … 45.170 s 10 runs
```
