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
```

Cargo.toml

@@ -38,7 +38,6 @@ backtrace = "0.3"
 base64 = "0.21"
 bitflags = "1.3"
 chrono = { version = "0.4", default-features=false, features = ["clock", "std"] }
-crossbeam-queue = "0.3"
 email = { git = "https://github.com/deltachat/rust-email", branch = "master" }
 encoded-words = { git = "https://github.com/async-email/encoded-words", branch = "master" }
 escaper = "0.1"
@@ -58,6 +57,7 @@ num-derive = "0.3"
 num-traits = "0.2"
 once_cell = "1.17.0"
 percent-encoding = "2.2"
+parking_lot = "0.12"
 pgp = { version = "0.9", default-features = false }
 pretty_env_logger = { version = "0.4", optional = true }
 qrcodegen = "1.7.0"