imap: Implemented APPEND final processing

The APPEND operation needs to be performed in several steps:
  1) We send "<tag> APPEND <mailbox> <flags> {<size>}\r\n"
  2) Server responds with continuation respose "+ ...\r\n"
  3) We start the transfer and send <size> bytes of data
  4) Only now we end the request command line by sending "\r\n"
  5) Server responds with "<tag> OK ...\r\n"

This commit performs steps 4 and 5, in the DONE phase, as more
processing is required after the transfer.