streaming: rename `git_istream` into `odb_read_stream`

streaming: rename `git_istream` into `odb_read_stream`

In the following patches we are about to make the `git_istream` more
generic so that it becomes fully controlled by the specific object
source that wants to create it. As part of these refactorings we'll
fully move the structure into the object database subsystem.

Prepare for this change by renaming the structure from `git_istream`
to `odb_read_stream`. This mirrors the `odb_write_stream` structure that
we already have.

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