Option to use variable length queue to avoid dropping message on burst

There is in this package a bunch of pressure release mechanisms to avoid breaking or using too much memory when things get clogged under load. The downside is that pubsub can drop some messages under load, even if the message did find its way across the network. This makes sense most of the time.

There is at least 3 spots where that can happen:

• validation queue (tunnable with pubsub.WithValidateQueueSize)
• outbound queue (tunnable with pubsub.WithPeerOutboundQueueSize)
• topic subscribe output queue (soon to be tunnable as well)

I believe some applications (like mine) would benefit from being able to tell pubsub to use as much memory as necessary and not drop messages in a heavy load scenario. Of course that mean that the application expose itself to being OOMed, but that can be easier to predict and handle than message semi-randomly disappearing.

Start by tracing the validation queue, outbound queue, and topic subscribe output queue, including the WithValidateQueueSize and WithPeerOutboundQueueSize options. Define how an option for retaining messages under burst load should apply across these pressure points, and verify that the configured behavior no longer drops messages while documenting the memory tradeoff.