odb/source-loose: wire up `write_object_stream()` callback

odb/source-loose: wire up `write_object_stream()` callback

Wire up the `write_object_stream()` callback.

Note that we don't move the implementation into "odb/source-loose.c".
This is because most of the logic to write loose objects is still
contained in "object-file.c", and detangling that requires us to do some
refactorings as explained in the preceding commit. So for now, the
implementation of writing an object stream is still located in
"object-file.c".

Signed-off-by: Patrick Steinhardt <ps@pks.im>
Signed-off-by: Junio C Hamano <gitster@pobox.com>