[Evolution-hackers] another locking change in CamelFolder

[Not Zed <notzed ximian com>]

Ok, this shouldn't affect connector but it might.

I've removed the folder lock on camel_folder_transfer_messages_to.  It
caused a deadlock.  Simple case, transfer messages from folder a to b
and while that is going, transfer messages from b to a.  Ugh.  Looks
like that original folder lock idea wasn't a good one, but it has served
ok so far.

IMAP is the only camel implementation that used it, and that didn't need
any changes (i think).

Anyway, this change, and some minor scheduling changes inside the mailer
make imap (in particular) a lot more interactive if you're doing a lot
of big moves, since it only locks each folder for each /get/append
operation rather than the whole task!

 Michael