Let us add the status variable innodb_buffer_pool_pages_LRU_freed
to monitor the number of pages that were freed by a buffer pool LRU
eviction scan, without flushing.

Also, let us simplify the monitor interface:
MONITOR_LRU_BATCH_FLUSH_COUNT, MONITOR_LRU_BATCH_FLUSH_PAGES,
MONITOR_LRU_BATCH_EVICT_COUNT, MONITOR_LRU_BATCH_EVICT_PAGES:
Remove.

MONITOR_LRU_BATCH_FLUSH_TOTAL_PAGE: Track buf_lru_flush_page_count
(innodb_buffer_pool_pages_LRU_flushed).

MONITOR_LRU_BATCH_EVICT_TOTAL_PAGE: Track buf_lru_freed_page_count
(buffer_pool_pages_LRU_freed).

Reviewed by: Vladislav Vaintroub