ref(server): Rename components dealing with envelopes

[jan-auer]

This cleans up long-standing legacy from when Relay only supported events but no envelopes. Most importantly, the actors for queueing and processing envelopes are now called EnvelopeManager and EnvelopeProcessor. Additionally:

• Renames the module declaring the envelope actors to actors::envelopes
• Renames error variants used for envelopes, and updates their Display impl
• Renames the global counter for in-flight envelopes to active_envelopes
• Renames accessor methods on relay_config::Config for the envelope buffer size
• Updates comments and logs around envelope processing in the envelopes module

#skip-changelog

[jan-auer]

I recommend reviewers to look at each commit separately.

[untitaker]

seems straightforward, this will create some new groups in s4s but nothing else