into n parts.

  Before this patch AHI search system was protected using single mutex named
  "btr_search_latch". It often use to cause contention resulting in performance
  hit.

  With this patch we are introducing ahi-partitioning that will split search
  systems into n parts and each index will be bound to a specified part based
  on its attributes (index_id and space_id).

  This will reduce contention and will help improve performance when AHI is
  heavily used.

  Reviewed by: Sunny Bains (sunny.bains@oracle.com)
  RB: 8773