Bug #12796: Record lost in HEAP table

Two handler objects were present, one was used for an insert and the other for a select
The state of the statistics was local to the handler object and thus the other handler
object didn't notice the insert.
Fix included:
1) Add a new variable key_stat_version added to whenever statistics was considered in need
of update (previously key_stats_ok= FALSE in those places)
2) Add a new handler variable key_stat_version assigned whenever key_stats_ok= TRUE was set
previously
3) Fix records_in_range to return records if records <= 1
4) Fix records_in_range to add 2 to rec_per_key to ensure we don't specify 0 or 1 when it isn't
and thus invoking incorrect optimisations.
5) Fix unique key handling for HEAP table in records_in_range

include/heap.h

@@ -136,6 +136,7 @@ typedef struct st_heap_share
   HP_KEYDEF  *keydef;
   ulong min_records,max_records;	/* Params to open */
   ulong data_length,index_length,max_table_size;
+  uint key_stat_version;                /* version to indicate insert/delete */
   uint records;				/* records */
   uint blength;				/* records rounded up to 2^n */
   uint deleted;				/* Deleted records in database */