r10299: remove the public (un)lock functions and introduce a transaction based

r10299: remove the public (un)lock functions and introduce a transaction based
private ldb API

ldb_sqlite3 is already working with this model and ldb_tdb will do
as soon as tridge finishes the tdb transaction code.

currently the transactions are always implicit and wrap any single
ldb API call except searching, the transaction functions are
currently not made public on purpose.

Simo.
(This used to be commit 1da4ac2cdcb7e54076f85242a93784260dced918)