imap: Send all pending tagged command replies before APPEND is started.

This probably doesn't matter a lot, but it's cleaner.

One side effect of this change is that if multiple APPENDs are pipelined
(without MULTIAPPEND) on the selected mailbox, this now performs a sync
between each APPEND and sends the EXISTS replies. This may be slightly
lower in performance, but it's also slightly more correct behavior.

src/imap/imap-commands.c

@@ -20,7 +20,11 @@ static const struct command imap4rev1_commands[] = {
 	{ "LOGOUT",		cmd_logout,      COMMAND_FLAG_BREAKS_MAILBOX },
 	{ "NOOP",		cmd_noop,        COMMAND_FLAG_BREAKS_SEQS },
 
-	{ "APPEND",		cmd_append,      COMMAND_FLAG_BREAKS_SEQS },
+	{ "APPEND",		cmd_append,      COMMAND_FLAG_BREAKS_SEQS |
+						 /* finish syncing and sending
+						    all tagged commands before
+						    we wait for APPEND input */
+						 COMMAND_FLAG_BREAKS_MAILBOX },
 	{ "EXAMINE",		cmd_examine,     COMMAND_FLAG_BREAKS_MAILBOX },
 	{ "CREATE",		cmd_create,      0 },
 	{ "DELETE",		cmd_delete,      COMMAND_FLAG_BREAKS_MAILBOX |