SQL-557: Update functions categorization

[nbagnard]

Update the functions categorization based on the JDBC spec. Started refactoring to split Mysql and MongoSql specific code since the supported functions are different.

[nbagnard]

Other changes made regarding the existing function list are :

• ATAN2 will now be listed as needing 2 arguments to match the MySql doc and sqlproxy support.
• ATAN will now be listed as accepting 1 or 2 arguments to match the MySql doc and sqlproxy support.
• SUBSTRING will now be listed as accepting 2 or 3 arguments to match the Mysql doc and sqlproxy support.
• CURRENTDATE function listed is not supported. CURRENT_DATE is.
• Same for CURRENTTIMESTAMP. Changed to CURRENT_TIMESTAMP.
• Same for FROMDAYS. Changed to FROM_DAYS.
• Same for FROMUNIXTIME. Changed to FROM_UNIXTIME.
• Same for UNIXTIMESTAMP. Changed to UNIX_TIMESTAMP.
• Same for CHARACTERLENGTH. Changed to CHARACTER_LENGTH.

[nbagnard]

Note : MongoDatabaseMetaDataTest > testGetFunctions() is failing because there are now 2 more functions reported. ATAN with 2 arguments and SUBSTRING with 3 arguments. I'll update the expected results once Matt's changes are merged.

[nbagnard]

nit: I would prefer if we named the classes MySQLFunctions and MongoSQLFunctions, but otherwise looks great!

Sure, I used to always use the parent class name as a suffix, but I agree it doesn't bring much here. Just makes the class names very long.