imap: Introduced FETCH_FINAL state for processing final fetch responses

A typical FETCH response can be broken down into four parts:

  1) "* <uid> FETCH (<what> {<size>}\r\n", using continuation syntax
  2) <size> bytes of the actual message
  3) ")\r\n", finishing the untagged response
  4) "<tag> OK ...", finishing the command

Part 1 is read in imap_fetch_resp(), part 2 is consumed in the PERFORM
phase by the transfer subsystem, parts 3 and 4 are currently ignored.