odb: update `struct odb_write_stream` read() callback

odb: update `struct odb_write_stream` read() callback

The `read()` callback used by `struct odb_write_stream` currently
returns a pointer to an internal buffer along with the number of bytes
read. This makes buffer ownership unclear and provides no way to report
errors.

Update the interface to instead require the caller to provide a buffer,
and have the callback return the number of bytes written to it or a
negative value on error. While at it, also move the `struct
odb_write_stream` definition to "odb/streaming.h". Call sites are
updated accordingly.

Signed-off-by: Justin Tobler <jltobler@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>