Move line handling into separate function
Handle :get-mailbox-tables in state :existing

This fixes a bug when opening a mailbox while trying to connect to the
server.  Now the mailbox contents is shown regardless of the connection
state.
Don't crash when syncing non-existent pending flags

This might happen when marking a message as read, even though it was
already marked as read (e.g. by viewing a read message), in a mailbox
where no other flag changes had been made through BIC.
Time out connection attempts after 60 seconds