assert on UTF-8 columns

Problem:
=======
(1) Multi-byte character cases are not considered during prefix index
cluster optimization check. It leads to fetch of improper results during
read operation.
(2) Strict assert in row_sel_field_store_in_mysql_format_func and it asserts
for prefix index record to mysql conversion.

Solution:
========
(1) Consider the case of multi-byte character during prefix index
cluster optimization check.
(2) Relax the assert in row_sel_field_store_in_mysql_format_func to allow
prefix index record to mysql format conversion.

The patch is taken from
https://github.com/laurynas-biveinis/percona-server/commit/1eee538087ffcf121c37f844b447ba5480faf081