odb/transaction: make `write_object_stream()` pluggable

How an ODB transaction handles writing objects is expected to vary
between implementations. Introduce a new `write_object_stream()`
callback in `struct odb_transaction` to make this function pluggable.
Rename `index_blob_packfile_transaction()` to
`odb_transaction_files_write_object_stream()` and wire it up for use
with `struct odb_transaction_files` accordingly.

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

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