AliSQL: [Feature] Issue#19 BUFFER POOL LIST SCAN OPTIMIZATION

Description:

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backport from WebScaleSQL

This patch includes:

--- backport of upstream work around buffer pool list scan.

     WL#7047 - Optimize buffer pool list scans and related batch processing code

     Reduce excessive scanning of pages when doing flush list batches. The

     fix is to introduce the concept of "Hazard Pointer", this reduces the

     time complexity of the scan from O(n*n) to O(n).

     The concept of hazard pointer is reversed in this work.  Academically a

     hazard pointer is a pointer that the thread working on it will declare as

     such and as long as that thread is not done no other thread is allowed to

     do anything with it.

     In this WL we declare the pointer as a hazard pointer and then if any other

     thread attempts to work on it, it is allowed to do so but it has to adjust

     the hazard pointer to the next valid value. We use hazard pointer solely for

     reverse traversal of lists within a buffer pool instance.

     Add an event to control the background flush thread. The background flush

     thread wait has been converted to an os event timed wait so that it can be

     signalled by threads that want to kick start a background flush when the

     buffer pool is running low on free/dirty pages.

--- fix for mysql bug#71411

     buf_flush_LRU() returns the number of pages processed. There are

     two types of processing that can happen. A page can get evicted or

     a page can get flushed. These two numbers are quite distinct and

     should not be mixed.