[BUGFIX] Do not serialize singleton properties on __sleep (Revision 1)

Serializing a Flow managed object will not serialize injected
properties as they will be reinjected anyway. This now works
consistently for all injections.

This also is a workaround for the PHP bug:
https://bugs.php.net/bug.php?id=65967

As the only reason we serialize an SplObjectStorage curently is
that we serialize Logger instances.

Change-Id: I1ad1a13610cedbff09ffaf3cd3a385b851c0d625
Releases: master, 2.1
Revision-Id: bdd54d18813591977ab5556cbe5948d6421262c1