Configure MongoDB collection (index) TTL

[a-filimonov]

We need to set a TTL for a collection stored in MongoDB for Mailhog.

This can easily be done here:

storage/mongodb.go

Line 24 in 6d871fb

err = session.DB(db).C(coll).EnsureIndexKey("created")

Like this:

	err = session.DB(db).C(coll).EnsureIndex(Index{Key:"created",ExpireAfter:60})

where 60 is the TTL in seconds