Active Record flexible batch operations(Insert,Update,Delete)

[ziaratban]

Q	A
Is bugfix?	no
New feature?	yes
Breaks BC?	no
Tests pass?	yes

MyActiveRecord::$batchInsertSize = 1000;
foreach($myData as $data){
    $obj = new MyActiveRecord();
    $obj->field1 = 'data1';
    $obj->field2 = 'data2';
    $obj->field3 = 'data3';
    $obj->batchSave(); #or batchInsert();
    #no any database operation execute until count of insert operation equal MyActiveRecord::$batchInsertSize
}
MyActiveRecord::flushBatchInsert(); // executing the remaining operations. required.

or batchDelete() or batchUpdate

[samdark]

@ziaratban tests seems to fail: https://travis-ci.com/yiisoft/yii2-mongodb/jobs/293832598