streaming: drop the `open()` callback function

streaming: drop the `open()` callback function

When creating a read stream we first populate the structure with the
open callback function and then subsequently call the function. This
layout is somewhat weird though:

  - The structure needs to be allocated and partially populated with the
    open function before we can properly initialize it.

  - We only ever call the `open()` callback function right after having
    populated the `struct odb_read_stream::open` member, and it's never
    called thereafter again. So it is somewhat pointless to store the
    callback in the first place.

Especially the first point creates a problem for us. In subsequent
commits we'll want to fully move construction of the read source into
the respective object sources. E.g., the loose object source will be the
one that is responsible for creating the structure. But this creates a
problem: if we first need to create the structure so that we can call
the source-specific callback we cannot fully handle creation of the
structure in the source itself.

We could of course work around that and have the loose object source
create the structure and populate its `open()` callback, only. But
this doesn't really buy us anything due to the second bullet point
above.

Instead, drop the callback entirely and refactor `istream_source()` so
that we open the streams immediately. This unblocks a subsequent step,
where we'll also start to allocate the structure in the source-specific
logic.

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