Revisit NAKACK#handleMessage and see if we can:

a) lower contention on NRW lock by not attempting to propel up oob messages one-by-one

I have no profiling data so I am speaking without a solid proof to back my claims. I am envisioning both oob and regular threads contending for a window as they push messages up the stack. oob threads are always going to attempt to lock per message (line 788). In an event that we received missing oob message in a series of oob messages we are going to repeatedly lock window per message traversed. Not good. We don't need this IMHO. I really like lock efficient removeMany algorithm. Let removeMany propel messages up the stack as much as possible, even oob messages.

b) why are we propelling oob messages on a timer (line 833)? Is it to lower the workload of the current thread, or is there a more fundamental reason?

c) undelivered_msgs is complicating things unnecessarily IMHO. Why don't we let oob threads go into removeMany code and remove as much messages as they can - both oob and regular and push them up rather than lock NRW per message?