Rename default pipeline or ask users to set it

The default pipeline in LS is named as `main`. This was an ok choice back then, given the understanding we had then about multi-pipeline and other future features. With the pipeline viewer project and multipipelines landing in 6.x, we think it makes sense to rethink our naming of the default pipeline.

The idea is to force the user to change the pipeline name to something more descriptive of what it does, like `apache`, `cloudwatch_logs` or so, but it is really hard to choose a name without knowing the pipeline's context. The worry here is that _every_ pipeline would be named `main`, which makes troubleshooting in the UI really hard. There are a couple of options:

1. Force user to set a pipeline name, without which LS wouldn't start. This change is pretty disruptive and many many folks have been doing `-f` and `-e`. Does it mean we have to add an option to set the pipeline name in the config? Or as a `--pipeline.name` CLI option?
2. Rename `main` to `_default_pipeline_name_` so it becomes _really_ obvious this needs to be changed. This is kinda analogous to how ES names its cluster name as `my-application`. It also makes it glaringly obvious in the UI (pipeline listing) if the name is `_default_pipeline_name_`.

### Breaking change

The rename (regardless if we do it now, or if a user chooses to do it at any point) is not simple since we have the pipeline name as a namespace for PQ and DLQ. For example, it is now `path.data/main/queue`. Changing this will mean users would have to drain their queue and then rename it. This point (thanks for raising it @robbavey) makes option #1 compelling.

Thoughts?