vcl/session: add api for changing session app worker

vcl/session: add api for changing session app worker

In case of multi process apps, after forking, the parent may decide to
close part or all of the sessions it shares with the child. Because the
sessions have fifos allocated in the parent's segment manager, they must
be moved to the child's segment manager.

Change-Id: I85b4c8c8545005724023ee14043647719cef61dd
Signed-off-by: Florin Coras <fcoras@cisco.com>