Patch: fetch ENVELOPE instead of headers

[Sergio Villar Senin <svillar igalia com>]

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Hi guys,

I have cooked a preliminary patch that allows tinymail to fetch the
ENVELOPE structure instead of fetching some headers. The reason to use
that is because we get (almost) the same information but it's far more
easy and fast for IMAP servers to return that information.

I'm not storing the ENVELOPE in the summary. As this is a first step I
just retrieve it and use it to store the same info in the summary than
the one fetched when downloading the headers.

It does not have many error checking code and it does not retrieve the
same info that we had with HEADERS but I got some nice figures about
performance. The difference in the case of GMail was minimum, they
probably cache also the HEADERS at server level, but for other servers
like AOL, downloading ~1200 headers lasted ~380 seconds while
downloading the envelope reduced that to ~230secs. So it seems a very
nice (and cheap) improvement.

I said that we didn't get the same information, and the reason is
because we don't get any information about attachments (we were
retrieving some non-standard but common used headers). Do you think we
should also retrieve BODYSTRUCTURE and check the presence of attachments
there?

Br