svr_n_page_hash_locks: Increase from 16 to 64. Before MDEV-15058,
we used to have the buf_pool.page_hash partitioned to each instance.

rw_lock_lock_word_decr(): Sleep a little in the spinloop.

rw_lock_s_lock_low(): Correct a comment. The function does perform
spinning.

This improves scalability in read-only workloads on a 32-CPU system
when the number of concurrent connections exceeds the CPU core count.

Thanks to Axel Schwenke for running benchmarks.