streaming: allocate stream inside the backend-specific logic

streaming: allocate stream inside the backend-specific logic

When creating a new stream we first allocate it and then call into
backend-specific logic to populate the stream. This design requires that
the stream itself contains a `union` with backend-specific members that
then ultimately get populated by the backend-specific logic.

This works, but it's awkward in the context of pluggable object
databases. Each backend will need its own member in that union, and as
the structure itself is completely opaque (it's only defined in
"streaming.c") it also has the consequence that we must have the logic
that is specific to backends in "streaming.c".

Ideally though, the infrastructure would be reversed: we have a generic
`struct odb_read_stream` and some helper functions in "streaming.c",
whereas the backend-specific logic sits in the backend's subsystem
itself.

This can be realized by using a design that is similar to how we handle
reference databases: instead of having a union of members, we instead
have backend-specific structures with a `struct odb_read_stream base`
as its first member. The backends would thus hand out the pointer to the
base, but internally they know to cast back to the backend-specific
type.

This means though that we need to allocate different structures
depending on the backend. To prepare for this, move allocation of the
structure into the backend-specific functions that open a new stream.
Subsequent commits will then create those new backend-specific structs.

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