(Backport)

Problem is that when insert (ha_start_bulk_insert) in i partitioned table,
it will call ha_start_bulk_insert for every partition, used or not.

Solution is to delay the call to the partitions ha_start_bulk_insert until
the first row is to be inserted into that partition


sql/ha_partition.cc:
  Bug#35845: unneccesary call to ha_start_bulk_insert for not used partitions
  
  Using a bitmap for keeping record of which partitions for which
  ha_start_bulk_insert has been called, and check against that if one
  should call it before continue with the insert/update, or if it has already
  been called.
  
  This way it will only call ha_start_bulk_insert for the used partitions.
  There is also a little prediction on how many rows that will be inserted into
  the current partition, it will guess on equal distribution of the records
  across all partitions, accept for the first used partition, which will guess
  at 50% of the given estimate, if it is a monotonic partitioning function.
sql/ha_partition.h:
  Bug#35845: unneccesary call to ha_start_bulk_insert for not used partitions
  
  Added help variables and function for delaying ha_bulk_insert until it has
  to be called.
  
  Fixed a comment.