Poll event 'kind' now format-validated and per-kind size-capped, wording of reserved-kind error changed

Internal poll events (used for delivering updates) now have stricter validation on their kind field:

• kind must match a pattern of a lowercase letter followed by lowercase letters, digits, or hyphens, up to 64 characters, and events that don't match are rejected with a new error message
• the maximum envelope size is now looked up per kind instead of using one fixed limit for all events
• the error message shown when a reserved kind is used has changed wording, now referring to a "trusted in-process producer" instead of a "server-authored producer"
• poll events can now optionally carry a media field