v5.5.0-beta.4

See full changelog here.

Commits:

• core: publish sendMessagesB (#949)
• xftp-server: fix stats (#944)
• agent: restart worker when it terminated (#948)
• control: try deleting from senders first (#950)
• xftp-server: add control port (#951)
• agent: send errors on subscription timeouts (#937)
• 5.5.0.4