make argument of handler::ha_write_row() const

int handler::ha_write_row(uchar * buf);
This interface clearly states that storage engine is allowed to modify memory where buf points.

It was used in native InnoDB system versioned tables and I fixed two bugs on that topic. One is MDEV-19486 and another one is not mentioned in Jira.

Actually, calling code for `handler::ha_write_row()` is not prepared to buffer change by engine. I see not places in the code where it's used now. Thus I see no reason on having a mutable argument here.

I'll make it const to prevent future bugs.