Because a `#connection_state` attribute will be added, I'd like to
consistently name the connection states everywhere they are used.

Most importantly, this documents the scenarios that need extra care to
avoid memory leaks:
* Commands such as #list or #fetch can have an enormous number of
  responses.
* Commands such as #fetch can result in an enormous size per response.
* Long-lived connections will gradually accumulate unsolicited server
  responses, especially +EXISTS+, +FETCH+, and +EXPUNGE+ responses.
* A buggy or untrusted server could send inappropriate responses, which
  could be very numerous, very large, and very rapid.