Every transaction was reading page 1 from the WAL to check the schema cookie
in op_transaction, causing unnecessary WAL lookups.
This commit caches the schema_cookie in Pager as AtomicU64, similar to how
page_size and reserved_space are already cached. The cache is updated when the
header is read/modified and invalidated in begin_read_tx() when WAL changes
are detected from other connections.
This matches SQLite's approach of caching frequently accessed header fields to
avoid repeated page 1 reads. Improves write throughput by 5% in our
benchmarks.
5dd503b7b9