Local inter-pipeline communication

[lquerel]

To support more complex deployment modes, we need to support a local inter-pipeline communication mechanism, whether pipelines belong to the same group or not. Many scenarios can be supported with such a mechanism, for example:

• A pipeline receiving traffic and splitting it across n specific pipelines based on a tenant ID.
• A pipeline receiving traffic and load balancing it across n instances of the same pipeline, to support platforms where SO_REUSEPORT is not effective.

To support this kind of inter-pipeline communication, we want to introduce the concept of named channels and add a new pair of exporter and receiver that reference a named channel, thereby establishing inter-pipeline communication. In the longer term, we also want to allow defining a class for these inter-pipeline channels such as:

• in-memory channel class
• quiver channel class