The handler function for reading one row from a specific index
was not optimized in the partitioning handler since it
used the default implementation.

No test case since it is performance only, verified by hand.

sql/ha_partition.cc:
  Implemented a optimized version of index_read_idx_map
  for the case when find flag == HA_READ_KEY_EXACT,
  which is the common case.
sql/ha_partition.h:
  Declared ha_partition::index_read_idx_map