* First try making get_recipients use MailHeader (nice and functional)
* Get it to compile by using not-so-functional style
* Add "empty-from" test, drop unnecessary check for error; continue using addrparse_header() instead of addrparse()
* Try to use functional style, unfortunately, I can't get the compiler to accept it
* Do it imperative-style: Do not overwrite To with Cc and vice versa
* Use addrparse_header() once more
* Still addrparse_header()
* Clippy
* Fix compile errors in tests
* Fix typo
* Fix tests again ;-)
* Code style
* Code style; try a HashMap<addr: String, display_name: String> as an address list but I am not convinced
* Code style; Use Vec<SingleInfo> as address list
* Clippy
* Add tests
* Add another test
* Remove stale comments
If error is ignored, the message will never be fetched again, even if
there was a database write error.
dc_receive_imf itself is modified to ignore unrecoverable errors, to
prevent endless refetching of incorrect messages.
If precheck_imf fails to check if message with the same rfc724_mid
already exists, the same message may be downloaded twice. Instead,
abort the whole operation and retry later.
Now 0 means "never delete", 1 means "delete at once" and other values
indicate the number of seconds after which them message should be
deleted from the server.
Configuration value interpretation is moved into
Context.get_config_delete_server_after() function.
Mutable UID reference is removed. Instead, ImapActionResult is
returned immediately.
If message has changed and UID does not correspond to expected
message, ImapActionResult::Failed is returned. Temprorary IMAP errors
result in ImapActionResult::RetryLater.
On the job side, ImapActionResult::RetryLater does not result in
immediate job retry anymore. Instead, job is retried with a backoff.
Envelope has the same Message-ID, but using other fields from the
envelope, such as From field, is error-prone. They may contain values
different from the message body. As we don't parse the envelope later on,
it is better not to fetch it during prefetch too.
This capability is not used by Delta Chat. Moreover, XLIST is deprecated
in favor of https://tools.ietf.org/html/rfc6154, which should be
supported instead. Delta Chat already looks for \Sent attribute in
get_folder_meaning().
