tdb2: simplify logging levels, rename TDB_DEBUG_* to TDB_LOG_*

tdb2: simplify logging levels, rename TDB_DEBUG_* to TDB_LOG_*

It was never clear to me which levels should be used for what cases.
I can only usefully distinguish three at the moment:
(1) TDB errors, which render the TDB unreliable.
(2) TDB user errors, caused by API misuse.
(3) TDB notifications of strange behaviour, from which we have recovered.