sql: organize connection pool as a stack rather than a queue

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
```

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ### Changes
 - use transaction in `Contact::add_or_lookup()` #4059
+- Organize the connection pool as a stack rather than a queue to ensure that
+  connection page cache is reused more often. #4065
 
 ### Fixes