When issuing commands in rapid succession, it was possible for
node-imap to send multiple "DONE" commands. This resulted in wedged
state when the server dropped out of IDLE and then didn't know what to
do with the second DONE command.
This change adds a safeguard that ensures _send never sends more than
one DONE, and queues (non-DONE/IDLE) commands up for when the server
was able to acknowledge.