tdb2: rearrange tdb_context layout so tdb1_context can match

By moving all the parts of struct tdb_context which logging and
locking use to the beginning of the structure, we can make
tdb1_context match, and thus pass that directly to the tdb_logerr()
function, and later the locking functions.

This is only necessary until we unify tdb1_context and tdb_context.