Pinned & Overflow Consumers

[Jarema]

This is a draft implementation for https://github.com/nats-io/nats-architecture-and-design/blob/main/adr/ADR-42.md

From the ADR:

We have a class of feature requests that all come down to adding behaviours on a consumer that affects delivery to groups of clients who interact with the consumer.

Some examples:

A single client should receive all messages from a consumer and if it fails another should take over
Groups of related clients should be organised such that certain clients only receive messages when various conditions are met
Groups of clients should access a Consumer but some should have higher priority than others for receiving messages
A consumer should deliver messages to clients in a specific grouping such that related messages all go to the same client group
The proposed feature here address some of these needs while paving the way for future needs to be addressed within this >framework and under this umbrella feature called Pull Consumer Groups.

The current focus is around providing building blocks that can be used to solve higher order problems client-side.

Draft status will be taken of after branch is cleared.

Signed-off-by: Tomasz Pietrek tomasz@nats.io