Do some cleanup of the classes in the Item_sum_hybrid class hierarchy,
which is where the MIN and MAX aggregate functions are implemented.

Item_sum_min and Item_sum_max have some functions that are very
similar, only differing in whether they use greater than or less than
when comparing values. Moved these functions to Item_sum_hybrid to
reduce duplication.

Replaced Item_sum_hybrid::cmp_sign (an int) with a bool which tells if
the instance represents the MIN or the MAX function.

Restructured the min_max_update_*_field functions so that they all
have the same structure. Also made sure that store() was only called
if a new min/max value was seen. (Previously, some of them stored a
value on every call.)

Make all of Item_sum_hybrid's member variables private.

Change-Id: Ic650b2338a1348d7cdf5f78ccc9fc5e65032ddae