v0.9.7: One message at a time

Features and improvements

• The semantics of message delivery got improved: agents now process a single
  message at a time, thus matching transactional boundaries.

• BREAKING: CSV Agent in 'serialize' mode handles only a single message at a
  time and can't aggregate multiple 'row' messages into a single CSV. Old
  behavior wasn't exactly deterministic, a suggested way would be to use
  Digest Agent to aggregate messages and then serialize them with
  CSV Agent.

Chores

• Update nokogiri (CVE-2020-7595).
• Update rack (CVE-2019-16782).
• Update puma (CVE-2019-16770).
• Fix regression for 'Delete Messages' button.