s4:schannel merge code with s3

s4:schannel merge code with s3

After looking at the s4 side of the (s)channel :) I found out that it makes
more sense to simply make it use the tdb based code than redo the same changes
done to s3 to simplify the interface.

Ldb is slow, to the point it needs haks to pre-open the db to speed it up, yet
that does not solve the lookup speed, with ldb it is always going to be slower.

Looking through the history it is evident that the schannel database doesn't
really need greate expanadability. And lookups are always done with a single
Key. This seem a perfet fit for tdb while ldb looks unnecessarily complicated.

The schannel database is not really a persistent one. It can be discared during
an upgrade without causing any real issue. all it contains is temproary session
data.