In so-called optimistic buffer pool lookups, we must not
dereference a block descriptor before we have made sure that
it is accessible. While buf_pool_t::resize() is running,
block descriptors could become invalid.

The buf::Block_hint class was essentially duplicating
a buf_pool.page_hash lookup that was executed in
buf_page_optimistic_get() anyway. For better locality of
reference, we had better execute that lookup only once.

buf_page_optimistic_fix(): Prepare for buf_page_optimistic_get().
This basically is a simpler version of Buf::Block_hint.

buf_page_optimistic_get(): Assume that buf_page_optimistic_fix()
has been called and the page identifier verified. Should the block
be evicted, the block->modify_clock will be invalidated; we do not
need to check the block->page.id() again. It suffices to check
the block->modify_clock after acquiring the page latch.

btr_pcur_t::old_page_id: Store the expected page identifier
for buf_page_optimistic_fix().

btr_pcur_t::block_when_stored: Remove. This was duplicating
page_cur_t::block.

btr_pcur_optimistic_latch_leaves(): Remove redundant parameters.
First, invoke buf_page_optimistic_fix() on the requested page.
If needed, acquire a latch on the left page. Finally, acquire
a latch on the target page and recheck the block->modify_clock.
If the page had been freed while we were not holding a page latch,
fall back to the slow path. Validate the FIL_PAGE_PREV after
acquiring a latch on the current page. The block->modify_clock
is only being incremented when records are deleted or pages
reorganized or evicted; it does not guard against concurrent
page splits.

Reviewed by: Debarun Banerjee