r13407: Change the credentials code to be more like the Samba4 structure,

r13407: Change the credentials code to be more like the Samba4 structure,
makes fixes much easier to port. Fix the size of dc->sess_key to
be 16 bytes, not 8 bytes - only store 8 bytes in the inter-smbd
store in secrets.tdb though. Should fix some uses of the dc->sess_key
where we where assuming we could read 16 bytes.
Jeremy.
(This used to be commit 5b3c2e63c73fee8949108abe19ac7a448a033a7f)