change the hash used for hash unique

After MDEV-27653 the hash used for hash uniques was changed to be 32-bit. This is a portable value (unlike the old one) but 64-bit hash would've produced less collisions. Let's change the hash to be

• not using ulong and other non-portable data types
• 64-bit always, on all architectures
• generally providing good and uniform distribution
• fast

Old tables must keep working, of course. This is the third but supposedly not the last time we'll change the hash function, so let's store the used hash function in the extra2 area. It'll simplify future changes.

May be we can change all non-persistent hash tables in the server to use the new function.

This bug will not cause wrong answers, only affects performance (as there are more collisions).
MariaDB will remember if a hash key was created as 32 or 64 bit and continue to use the original length even if we change back the default to 64 bit at some point.