PS-10595 [8.4]: Lazy initialization for buffer pool block latches

[polchawa-percona]

Buffer pool initialization speedup

1. Refactor buf_block_init into buf_block_init_light(), performing only lightweight setup without creating latches.
2. Introduce lazy, latch initialization for buffer blocks via buf_block_initialize_latches().
3. Call lazy latch initialization when a block is taken from the free list the first time.
4. Adjust buf_pool teardown to free mutexes/rwlocks only for blocks that had latches initialized.

The patch is based on contribution from Alexey Bychko: adf7b21

Modifications:

• sysvar to enable this optionally,
• the lazy initialization is enough to be done when taking from the free list,
• sharding is removed (was broken),
• replaced CAS with regular store to non-atomic variable:
  • no concurrent accesses to that logic for the same block,
  • note, that otherwise: they wouldn't be thread-safe (a thread could have been scheduled out just before creating the mutex but after the CAS succeeded),
  • accesses are protected by free_list_mutex or it is a startup phase

[polchawa-percona]

Note - the alternative possible solution is: #6037
I will test both and provide numbers.