Added store_many_vectors on Mongo Storage

[etudor]

Because I found this method to be very useful on Redis storage, I've added it to the mongo storage as well.

I haven't done a benchmark to compare what is the speed increase vs single inserts.

[amorgun]

@etudor The test is broken because it cannot import InsertOne. You should add pymongo to test requirements.

[etudor]

@amorgun I have updated this

[amorgun]

@etudor It looks like a lot of mongo tests are broken now in python2.7. Please, check if it is related to your changes. Maybe you should pin an older version of pymongo.