r10759: make modules easier to write by allowing modules to only implement the

r10759: make modules easier to write by allowing modules to only implement the
functions they care about, instead of all functions. This also makes
it more likely that future changes to ldb will not break existing
modules
(This used to be commit 45f0c967b58e7c1b2e900a4d74cfde2a2c527dfa)