PS-10595 [9.7]: Lazy initialization for buffer pool block latches

[polchawa-percona]

https://perconadev.atlassian.net/browse/PS-10595

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 for the first time.
4. Adjust buf_pool teardown to free mutexes/rwlocks only for blocks that had latches initialized.

The patch is based on a contribution from Alexey Bychko: adf7b21

Modifications on top of the original contribution:

• a system variable to enable this optionally;
• the lazy initialization is done when a block is taken from the free list;
• sharding was removed (it was broken);
• replaced the CAS with a regular store to a non-atomic variable:
  • there are no concurrent accesses to this logic for the same block;
  • otherwise it would not be thread-safe (a thread could be scheduled out just after the CAS succeeded but before creating the mutex);
  • accesses are protected by free_list_mutex, or happen during the startup phase.