Support the STD agg. fnc. on MySQL databases

This was supported in past versions, preserving here.
Other drivers currently do not support it (as in the past).

server/store/adapters/rdbms/drivers/mysql/dialect.go

@@ -238,7 +238,7 @@ func (d mysqlDialect) ExprHandler(n *ql.ASTNode, args ...exp.Expression) (expr e
 		return drivers.OpHandlerNotIn(d, n, args...)
 	}
 
-	return ql.DefaultRefHandler(n, args...)
+	return ref2exp.RefHandler(n, args...)
 }
 
 func (d mysqlDialect) ValHandler(n *ql.ASTNode) (out exp.Expression, err error) {

server/store/adapters/rdbms/drivers/mysql/ql.go

@@ -0,0 +1,16 @@
+package mysql
+
+import (
+	"github.com/cortezaproject/corteza/server/store/adapters/rdbms/ql"
+	"github.com/doug-martin/goqu/v9/exp"
+)
+
+var (
+	ref2exp = ql.ExprHandlerMap{
+		"std": {
+			Handler: func(args ...exp.Expression) exp.Expression {
+				return exp.NewSQLFunctionExpression("STD", args[0])
+			},
+		},
+	}.ExprHandlers()
+)

server/store/adapters/rdbms/ql/ref.go

@@ -280,6 +280,12 @@ var (
 				return exp.NewSQLFunctionExpression("MAX", args[0])
 			},
 		},
+
+		"std": {
+			HandlerE: func(args ...exp.Expression) (exp.Expression, error) {
+				return nil, fmt.Errorf("STD function not supported on this database")
+			},
+		},
 	}
 )
 
@@ -304,6 +310,10 @@ func (ee ExprHandlerMap) RefHandler(n *ql.ASTNode, args ...exp.Expression) (exp.
 		return nil, fmt.Errorf("unknown ref %q", r)
 	}
 
+	if ee[r].Handler != nil && ee[r].HandlerE != nil {
+		panic("invalid ql ref definition: both Handler and HandlerE are set")
+	}
+
 	if ee[r].Handler != nil {
 		return ee[r].Handler(args...), nil
 	}