Add an internal API to stream content objects

Add an internal API to stream content objects

For future delta work where we do more interesting things than just
"tar of new objects", this lays the groundwork for doing streaming
writes into content objects.

It's also more efficient, as we avoid many intermediate allocations
and virtual calls.  Just a single `g_output_stream_write_all` for the
splice case.

Conflicts:
	src/libostree/ostree-repo-private.h
	src/libostree/ostree-repo-static-delta-processing.c