Document behavior with file-based peers after 2.8

[kentquirk]

Refinery 2.7 added peer-to-peer communications over Redis and added some limited features (shared stress, etc) that use it.

File-based peers still work, but don't support those pubsub features, and we didn't talk about that in the 2.7 release notes. We need to add more info about it in the 2.8 release, because there are now going to be more things (redistribution of traces on shutdown) that won't work.

We need to:

• Make DisableRedistribution be true whenever file-based peers are in use
• Document what features you don't get with them

This is not a breaking change, but it means that orgs using file-based peers are going to miss out on the scalability features in 2.8.

We don't have a good estimate of how many people use file-based peers for clusters with more than one node, and we don't really know whether not using Redis is a requirement or a preference.

In 2.9, we might be in a position to implement pubsub over the file-based peer system. Or we can decide to implement one or more other pubsub systems as an alternative to Redis, and insist that one of them be used.