Create new connections for each new collection/user.

[dmassart]

Fixes issue #33 by creating a new connection pool for each new collection/user. Note that, as there is no way to detect when a collection is not used anymore (cf. issue #29), it is necessary to schedule a job for explicitly close collections and connections when tuktu runs for a long period of time. This can be done using the MongoDB Cleanup processor.

[ErikTromp]

This is not how pooling should work. I will start working on a solution that fixes all Mongo-related issues.