Split object_space_dump into smaller messages

Closes #52 

Merge after #76 is merged.

Stuff that changed : 

1. Added snapshot_no to obj space dump events (so these can be grouped even if they arrive in different messages)
2. Accumulated events will be sent prematurely(before the 1 second gap) if we have more than a fixed number of events in the queue (set to 400 for now after some experimentation)
3. Added a limit of 20 object dumps to be grouped into one event. So we'll at a time receive a max of 20*400 object dumps in one zmq message.
I think this number has to be way higher, but it has to be calibrated. But something like 1000 should not be too bad of a limit IMO.