Fix up regr functions in regrmysql.cpp so regr_* funtions work correcly for InnoDB tables

regrmysql.cpp is the UDAF plugin for the regr_*** functions. It hasn't been kept up to date.

The idea is that we also add the regr_*** functions to all engines via the Server API.

However, the algorithms haven't been kept up with the changes in the Columnstore implementations. In addition, they return a fixed decimal of 2 decimal places, which isn't correct.