imap: Fix potential hang if client disconnects with pipelined ambiguous commands

imap: Fix potential hang if client disconnects with pipelined ambiguous commands

For example if client pipelines FETCH+LOGOUT commands and disconnects during
a large FETCH output, the LOGOUT command is processed as waiting for
unambiguity. This code path however doesn't detect a client disconnection,
and input IO handler is removed so it can't detect it either. So the imap
process hangs.