Glacier2 router and flow control

[bernardnormier]

The Glacier2 router currently (3.7) does not provide a flow control mechanism: if the forwarded-to server or client (for callbacks) is slow, requests are queued in the router. There is no limit to the depth of this queue, and no way to apply back pressure on the sender. This is true in buffered and unbuffered mode.

This issue is most acute for oneway requests. For two-way requests, the sender can see that responses are not received or take a long time, and possibly take action.