Update the syslink documentation. A number of major design changes have

Update the syslink documentation.  A number of major design changes have
been made.  In particular, there was insufficient uniqueness in the logical
id methodology in the face of any major (live) change made in the cluster
topology.  Introduce a session id abstraction to deal with the issue.
This also allows us to avoid adding another sysid_t to the message structure
(there are already three).