- Added more comments.
- Added a RANGE_OPT_PARAM::remove_jump_scans flag that disables construction of index_merge
  SEL_TREEs that represent unusable conditions like "key1part1<c1 OR key2part2<c2"
- make prune_partitions() function handle the case where range analysis produces a list of 
  index_merge trees (it turned out that this is possible, appropriate test case added).
- Other small fixes.


mysql-test/r/partition_pruning.result:
  WL#2985 "Partition Pruning": post-review fixes: more test cases
mysql-test/t/partition_pruning.test:
  WL#2985 "Partition Pruning": post-review fixes: more test cases
sql/opt_range.cc:
  WL#2985 "Partition Pruning": post-review fixes:
  - Added more comments.
  - Fix the debug printouts
  - Added a RANGE_OPT_PARAM::remove_jump_scans flag that disables construction of index_merge
    SEL_TREEs that represent unusable conditions like "key1part1<c1 OR key2part2<c2"
  - make prune_partitions() function handle the case where range analysis produces a list of 
    index_merge trees (it turned out that this is possible, appropriate test case added).
sql/sql_partition.cc:
  WL#2985 "Partition Pruning": post-review fixes: make requested edits in comments.
sql/table.h:
  WL#2985 "Partition Pruning": post-review fixes: added bool TABLE::no_partitions_used
  (this change was missed when making the original cset)