[NFR]MongoCollection Uniqueness and Phalcon\Validation support

[Jurigag]

In phalcon 3.1.0 there will be added Phalcon\Validation support for Phalcon\Mvc\Collection phalcon/cphalcon#12445. Since that MongoCollection need some changes to make it work with Phalcon\Validation too as well with Uniqueness validator(DIRTY states).

~Any idea how to solve it ? Should there be whole different Uniqueness class for MongoCollection in incubator ? Or i will just add proper PR with class_exists(in uniqueness validator) and instanceof check after someone will make MongoCollection working with validation and Uniqueness validator ?~~~

Well MongoCollection extends Phalcon collection so whatever.

Also if someone know Mongo well enough and is using it could check Uniqueness source if it's correct in this PR to work with Mongo correctly. I guess will add tests to check all cases anyway.

[Jurigag]

Before we release phalcon 3.1.0 i think this should be implemented i think @sergeyklay

[Jurigag]

Implemented in 3.1.x branch