object-store: move alt_odb_list and alt_odb_tail to object store

object-store: move alt_odb_list and alt_odb_tail to object store

In a process with multiple repositories open, alternates should be
associated to a single repository and not shared globally. Move
alt_odb_list and alt_odb_tail into the_repository and adjust callers
to reflect this.

Now that the alternative object data base is per repository, we're
leaking its memory upon freeing a repository. The next patch plugs
this hole.

No functional change intended.

Signed-off-by: Stefan Beller <sbeller@google.com>
Signed-off-by: Jonathan Nieder <jrnieder@gmail.com>
Signed-off-by: Nguyễn Thái Ngọc Duy <pclouds@gmail.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>