PROBLEM
-------

1. ibuf_contarct_in background() function is trying to merge
   all pages related to a certain table (which contains no record).
   while in background heavy DML operations are happening.
2. When contracting the pages in ibuf_merge_space() we call
   ibuf_get_merge_pages() to get the pages to be merged.This function
   call returns the volume ( in bytes) of pages to be merged.
3. In our case , the returned volume is zero since there are no pages
   to be merged ,but we increment the volume returned by 1 .
4. This volume is returned to ibuf_contract_in_background() which
   assumes that some pages have been merged and doesn't break the loop
   It doesn't check if any pages have been merged but assumes that since
   volume is non zero some pages are merged and waits for it to reach
   sum_pages and gets stuck in this loop

FIX
---

1) The reason we increment the volume by 1 ,is because the function
   is supposed to return zero only if the ibuf tree is empty.
2) To fix this we first have delinked the ibuf_merge_space() from the loop
   present in ibuf_contarct_in background() , it is independently called
   during quisec and returns number of pages merged instead of the size.
3) Also as part of this fix we have backported Bug19724300 to make used of space_id
   instead of table id.

    Bug#19724300 REMOVE TABLE LOOKUPS FROM THE CHANGE BUFFER

    When exporting a tablespace, InnoDB needs to merge all buffered changes to
    that tablespace. However, the API for that is using the table_id, and we are
    performing unnecessary table lookups in the change buffer. The lookups should
    never fail (there is dead code), because MDL will be preventing a concurrent
    ALTER/TRUNCATE/DROP TABLE while the export operation is pending.

    ibuf_get_table(): Remove.

    ibuf_merge(): Replace table_id with space_id.

    ibuf_contract(): Call ibuf_merge_pages() directly.

    ibuf_merge_in_background(): Renamed from ibuf_contract_in_background().
    Replace table_id with space_id.

[#rb12209 AND #rb6890  approved by jimmy ]