readding socket.io mongodb adapter

[oplik0]

Adds the now official mongodb adapter for socket.io, and some basic code for handling clustering with just mongodb.

Redis is still probably preferred, but sometimes running it might require additional costs (@Antosik you probably made your setup work already, but if you're still interested in running without Redis it should be possible now), and it seems people are interested in possibility of running without it.

Additionally, due to this and #9916 I added current version MongoDB Replica Set tests, but I later realized that there are no tests for clustering (so I was just testing it manually). I can remove them or let them stay for the transactions and also just as a way of ensuring things work with rs setups and on current mongo versions.
Their weird name is due to not wanting to modify existing tests that check for the database matrix starting with mongo - so mongo-rs was out.