Problem was that ha_partition::records_in_range called
records_in_range for all non pruned partitions, even if
an estimate should be given.

Solution is to only use 1/3 of the partitions (up to 10) for
records_in_range and estimate the total from this subset.
(And continue until a non zero return value from the called
partitions records_in_range is given, since 0 means no rows
will match.)

sql/ha_partition.cc:
  Bug#48846: Too much time spent in ha_partition::records_in_range if not able to prune
  
  estimate_rows_upper_bound and records_in_range are very similar
  (the only difference is the function and its parameters to use)
  so I created a common function for this.
  
  Since these calls from the optimizer are only estimates, it is
  not neccesary to call them for every partition, it can use
  a much smaller subset of the used partitions instead,
  which improves performance for selects.
sql/ha_partition.h:
  Bug#48846: Too much time spent in ha_partition::records_in_range if not able to prune
  
  Added two private functions to help some
  optimizer calls.