Motivation:
client session slot handling is not only used by call-back but as well
by a regular client and proxy-io client inside dCache. Thus, this functionality
should be shared to reduce code duplication.

Modification:
introduce ClientSession class that does the session slot management.

Result:
client side session slot management can be shared with other classes.

Acked-by: Paul Millar
Acked-by: Lea Morschel
Target: master