Pages allocated to AHI need to be subject to LRU

[Raghavendra D Prabhu]

Currently, the pages for AHI are allocated from buffer pool LRU
pages (with mem_heap_create_in_btr_search which allocates with
MEM_HEAP_BTR_SEARCH | MEM_HEAP_BUFFER).

However, once allocated, they are not freed unless the page which
which they are associated with (the index page which is hashed)
is also freed.

There already exists functions like buf_block_free and
buf_LRU_block_free_non_file_page, but they are not called in LRU
shrinker path.

It may be interesting to have it in the LRU path -- in
buf_LRU_free_block before trying to flush dirty blocks, since
hash index pages are clean and should be attempted to freed
before any I/O

Another implication is that this prevents buffer pool LRU pages
("Database pages" in innodb status)
decreasing with more and more AHI pages taking that memory.

[Shahriyar Rzayev (rzayev-sehriyar)]

Percona now uses JIRA for bug reports so this bug report is migrated to: https://jira.percona.com/browse/PS-2393