If a MailboxInfo has the attribute \Noselect,
it is not possible to use this name as a selectable mailbox.
Therefore it should not be passed to the directory handlers.
The issue pops up if one has a hierarchy like this:
INBOX
INBOX/lists/stuff
INBOX/lists/otherStuff
Even though lists is not a valid inbox (doesn't contain mail, only other maildirs)
it will show up in the directory listing, when we iterate over the MailboxInfo.
It does have the corresponding attribute set though and we can simply filter it out.
Unfortunately, the IMAP protocol hasn't been designed to be used from multiple
goroutines at the same time. For instance, if you fetch twice the same message
from two different goroutines, it's not possible to tell whether the response
is for one receiver or the other. For this reason, go-imap clients aren't safe
to use from multiple goroutines.
This commit changes the IMAP workers to be synchronous again (a command is
executed only after the previous one has completed). To use IMAP from different
threads, popular clients (e.g. Thunderbird) typically open multiple
connections.
