libctdb: more bool conversion, and accompany lock by ctdb_db in API

libctdb: more bool conversion, and accompany lock by ctdb_db in API

I missed some int->bool conversions previously, particularly the
return of ctdb_writerecord().

By always handing functions ctdb_connection or ctdb_db, we keep it
consistent with the rest of the API and can do extra lock consistency
checks.

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>