- Fix problems in the "Interval walking" partition interval analyzer.


mysql-test/r/partition_pruning.result:
  BUG#20257: Add more testcases for partition pruning
mysql-test/t/partition_pruning.test:
  BUG#20257: Add more testcases for partition pruning
sql/sql_partition.cc:
  BUG#20257: Fix partition pruning for BIGINT UNSIGNED, interval walking:
   - provide special handling for an edge case of interval of size 4G-1.
   - Store interval size in ulonglong, not uint (it was possible to miss partitions this way)
   - In get_next_partition_via_walking(), interpret the value of walked-over field as having the 
     same "signedness" as field (this is so because this value was obtained by calling field->val_int())
   - Remove out of date todo comment.