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.
Reviewed-by: Jussi Saurio <jussi.saurio@gmail.com>
Closes#3727
418fc90f8a