Fix problem with MongoDB

[stuehmer]

Tomcat crashes sometimes (irregularily) with this message:

SEVERE: The web application [/play] appears to have started a thread named [MongoCleaner1574466115] but has failed to stop it. This is very likely to create a memory leak.

There is a related Mongo issue https://jira.mongodb.org/browse/JAVA-306

[stuehmer]

Updating the mongo-java-driver (to a version >=2.11.0) helped resolve a similar issue in Tomcat: SHA: bec59e1

SEVERE: The web application [/play] created a ThreadLocal with key of
type [com.mongodb.DBTCPConnector$1] [...] but failed to remove it when
the web application was stopped. This is very likely to create a memory
leak.

... however the issue above remains.