Ensured that `self` (a `Node` [or a `Leaf`]) is always in a DB session
Found the cause for all our pains: The worker threads were synchronized because the `ZmqQueueThread` was using a `ThreadDevice` with the `REQ` and `REP` endpoints: These endpoints cause a strict *send* request, receive *corresponding* response (and block *all other* request from reaching there destination till the original request has been responded to). Using `ROUTER` and `DEALER` solved the problem: Thanks to them the workers can now work truly in parallel, since there is no request-response regime over *all* endpoints, but only for the corresponding ones.