[Evolution] POP changes, rewrite/etc [head]

[Not Zed <notzed ximian com>]

I've just checked in a new implementation of POP3 to the head version of
camel/evolution.

The main changes are:
 - downloads backed by a cache, which isn't often used but could save
time sometimes
 - asynchronous downloads, if the server reports it supports PIPELINING,
multiple commands are sent without waiting for replies.
 - removed KPOP authentication mechanism.  Reportedly it isn't used very
widely, and any that do use AUTH KERBEROS_V4 as well.
 - added support for SASL authentication mechanisms, although so far
only AUTH LOGIN tested.

Main difference - if your server reports it supports pipelined commands,
and you are on a slow, remote, link (i.e. high latency), you should see
significant performance increases if you are downloading lots of small
messages (like upto 5-10x faster overall, including the 'deleting
messages' part).

I've done a fair bit of testing, including cancellation and whatnot, but
no doubt something will have slipped through.  Maybe there should also
be an option to turn on pipelining even if the server doesn't say it can
do it (its been implemented in a way that works on many other servers
too).

Most of the progress reporting should have made it from the old code,
but if your server doesn't support UIDL, the process of creating uid's
isn't reported very well.  This doesn't affect many people, and this
step should be >> faster than it used to be anyway.

So just keep an eye out if you use pop, i'd like to know if there's any
problems like: 

 - mishandled downloads
 - cancellation misbehaviour
 - interoperability with existing configs
 - build problems
 - memory leaks, particularly in exceptional circumstances
 - etc.

Thanks,
 Michael