In the FLOAT/DOUBLE to string conversions a class global string was used
to store the result. Unfortunately it is possible for an instance of
this class to be used by multiple threads of PrimProc simultaneously.
This would cause a race and data corruption or more likely a crash.
This fix passes a string object from the caller to use instead.
Functions such as reverse() that do float/double to string conversion
use printf's %g to do it. Unfortunately this adds a '+' symbol before
the exponent symbol. MariaDB doesn't do this.
This patch builds the string in a way that does not have that problem,
it resembles the way it is done elsewhere in the codebase.
