Improve reliability of detecting upload available.

Detecting a !ready file being placed into the upload directory is
problematic, due to possible races in setting watches on a just created
directory.

So instead, we just watch for a file touched when the sftp session ends.

This means we more reliably detect when a !ready file has been uploaded,
at the cost of doing extra scans (in the probably rare case) when a sftp
session hasn't uploaded a !ready file.

calm/calm.py

@@ -814,7 +814,7 @@ def sighup(signum, frame):
                                     action |= Event.read_relarea
                             elif path.startswith(args.stagingdir) and (filename != 'tmp'):
                                 action |= Event.read_uploads
-                            elif (path.startswith(args.homedir)) and (filename == "!ready"):
+                            elif (path.startswith(args.homedir)) and (filename == ".sftp-session-close"):
                                 action |= Event.read_uploads
                         else:
                             # None means no more events are currently available, so