Rename native functions

[ngaro]

On newer versions of MySQL and MariaDB there are native functions that clash with udf_infusion defined functions.
The following renames where necessary to solve this:

• median() is now med()
• percentile_cont() is now percentile_contin()
• percentile_disc() is now percentile_discr()

[ngaro]

I just tested it with MySQL version 8.0.20. It doesn't seem to be a issue there. They are only native functions in MariaDB.

If renaming the functions is too big of a change then i would suggest creating a extra branch mariadb (and i'll change this pull request to that branch)

[vlabella]

did the mariadb branch ever get created to solve this issue?