Some calls to buf_read_ahead_linear() seem to be useless

InnoDB linear read-ahead (which is enabled when innodb_read_ahead_threshold is nonzero) is being invoked in the low-level function buf_page_get_low(). This does not seem useful for page accesses that are likely to be one-time-only, with no anticipation of accessing nearby pages in the near future.

We should not attempt to invoke read-ahead on index pages that are not at the leaf level.

We should not invoke linear read-ahead in functions that would essentially allocate or free pages, because allocated pages should be reinitialized by buf_page_create(), eliminating the need to read anything from the file system. Likewise, freeing pages should not involve accessing any sibling pages, except for freeing singly-linked lists of BLOB pages.

We should not invoke read-ahead in btr_cur_t::pessimistic_search_leaf() or in a pessimistic operation of btr_cur_t::open_leaf(), because pessimistic operations should be preceded by optimistic operations, which should already have invoked read-ahead.

By doing all this, we will hopefully improve performance in I/O bound workloads.