Remove persistent non-document reporting from spec. (#191)

* Remove persistent non-document reporting from spec.

This change removes the concepts which are only relevant to persistent
out-of-band reporting specs, such as Network Error Logging, and
explicitly defines reporting mechanisms for documents, where the
generated reports are queued with the document itself, and thereby
inherit its lifetime. Generic reporting concepts are extracted into
another section so that they can be used by other specs.

This removes endpoint weight and priority from the basic reporting spec,
and creates a 'network reporting endpoint' concept for network
reporting. Endpoint groups are similarly moved to network reporting, as
there is no longer any need to group endpoints otherwise. Retries and
failover are removed from basic reporting.

Addresses issues raised in #158.