SessionHandle: the protocol specific pointer is now a void *

SessionHandle: the protocol specific pointer is now a void *

All protocol handler structs are now opaque (void *) in the
SessionHandle struct and moved in the request-specific sub-struct
'SingleRequest'. The intension is to keep the protocol specific
knowledge in their own dedicated source files [protocol].c etc.

There's some "leakage" where this policy is violated, to be addressed at
a later point in time.