MongoDB: replaceOne operation

[jewertow]

References #2100

[seglo]

Looks like you're on the right track 👍

[jewertow]

@seglo what do you think about using in tests and docs object DomainObject(https://github.com/akka/alpakka/pull/2187/files#diff-670af5a1ee8a19bd56d7ed678837ecc6) instead of the existing Number.
In case of replaceOne object Number is useless.
Do you want to have all tests and examples using the DomainObject or only replaceOne?

In my opinion it is a little bit confusing that we declare the class Number with only one field _id and then we update fields value and updatedValue.
If you want to have unified tests and examples I can change it and prepare next pull request.
If you think that the DomainObject is not a good example you can suggest something else.

[jewertow]

Ok, the implementation is done. You can review the code.

[seglo]

@jewertow Thanks. I'm on vacation ATM, but @ennru or someone else from the team will take a look next week.

[ennru]

LGTM.

[ennru]

Thank you for another great contribution!