performance: replace a critical usage of Array.concat with for loop/push (#7175)

* performance this Array.concat with for loop/push

for loop/push is much faster than Array.concat.
For the largest/ugliest queries, this can get
called hundreds of thousands of times, so the
performance difference does matter.

* add message to changelog.md

changelog.md

 # Future
+- [PERFORMANCE] more efficient array handing for certain large queries [#7175](https://github.com/sequelize/sequelize/pull/7175)
 - [FIXED] Add `unique` indexes defined via options to `rawAttributes` [#7196]
 - [FIXED] Removed support where `order` value is string and interpreted as `Sequelize.literal()`. [#6935](https://github.com/sequelize/sequelize/issues/6935)
 - [CHANGED] `DataTypes.DATE` to use `DATETIMEOFFSET` [MSSQL] [#5403](https://github.com/sequelize/sequelize/issues/5403)

lib/model.js

@@ -1591,7 +1591,10 @@ class Model {
             // Force array so we can concat no matter if it's 1:1 or :M
             if (!Array.isArray(associations)) associations = [associations];
 
-            return memo.concat(associations);
+            for (let i = 0, len = associations.length; i !== len; ++i) {
+              memo.push(associations[i]);
+            }
+            return memo;
           }, []),
           _.assign(
             {},