- post-...-post review fixes
- Added "integer range walking" that allows to do partition pruning for "a <=? t.field <=? b"
  by finding used partitions for a, a+1, a+2, ..., b-1, b. 


mysql-test/r/partition_pruning.result:
  WL#2985 "Partition Pruning": tests for "integer range walking"
mysql-test/t/partition.test:
  WL#2985 "Partition Pruning": post-review fixes
mysql-test/t/partition_pruning.test:
  WL#2985 "Partition Pruning": tests for "integer range walking"
sql/handler.h:
  WL#2985 "Partition Pruning": "integer range walking": 
  - class partition_info now has pointers to "partitioning interval analysis" functions
  - added "partition set iterator" definitions.
sql/opt_range.cc:
  WL#2985 "Partition Pruning": "integer range walking":
  - Switched to use "partitioning interval analysis" functions
  - Fixed two problems in find_used_partitions() that occur on complicated WHERE clauses.
sql/sql_partition.cc:
  WL#2985 "Partition Pruning": "integer range walking": 
  - Added "partitioning interval analysis" functions: get_part_iter_for_interval_via_mapping, 
    get_part_iter_for_interval_via_walking, 
  - Added appropriate partition-set-iterator implementations
  - Added a function to set up Partitioning Interval Analysis-related fields in partition_info.
sql/sql_select.cc:
  WL#2985 "Partition pruning": added comments.