request LIST calls / mailbox and STATUS info by using pipelined imap-calls.

This means we do the imap-call and don't wait for the response and continue
with the next mailbox.
Because we remember the issued tags we are able to retrieve the
imap-responses and assign the response to the correct mailbox.

The advantage of this method is that we don't have to wait on each response.

git-svn-id: https://svn.code.sf.net/p/squirrelmail/code/trunk/squirrelmail@5159 7612ce4b-ef26-0410-bec9-ea0150e637f0