core: Switch SQLite to use locking_mode EXCLUSIVE

Let's switch the SQLite benchmarks to use similar file locking protocol
as we do. Improves SQLite's performance by 2x, but Limbo is still
faster.

Before:

rusqlite/Prepare statement: 'SELECT * FROM users LIMIT 1'
                        time:   [2.1027 µs 2.1239 µs 2.1563 µs]
                        thrpt:  [463.75 Kelem/s 470.83 Kelem/s 475.57 Kelem/s]

After:

rusqlite/Execute prepared statement: 'SELECT * FROM users LIMIT 1'
                        time:   [940.07 ns 944.16 ns 949.39 ns]
                        thrpt:  [1.0533 Melem/s 1.0591 Melem/s 1.0638 Melem/s]

core/benches/benchmark.rs

@@ -107,6 +107,8 @@ fn rusqlite_bench(criterion: &mut Criterion) {
 
     let conn = rusqlite::Connection::open("../testing/testing.db").unwrap();
 
+    conn.pragma_update(None, "locking_mode", &"EXCLUSIVE")
+        .unwrap();
     group.bench_function("Prepare statement: 'SELECT 1'", |b| {
         b.iter(|| {
             conn.prepare("SELECT 1").unwrap();