__MEMMOVE_SSSE3_BACK FROM STRING::COPY

Issue:
-----
While using row comparators, the store_value functions call
val_xxx functions in the prepare phase. This can cause
valgrind issues.

SOLUTION:
---------
Setting up of the comparators should be done by
alloc_comparators in the prepare phase. Also, make sure
store_value will be called only during execute phase.

This is a backport of the fix for Bug#17755540.