object-store: rename files to "odb.{c,h}"

In the preceding commits we have renamed the structures contained in
"object-store.h" to `struct object_database` and `struct odb_backend`.
As such, the code files "object-store.{c,h}" are confusingly named now.
Rename them to "odb.{c,h}" accordingly.

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

meson.build

@@ -396,8 +396,8 @@ libgit_sources = [
   'object-file-convert.c',
   'object-file.c',
   'object-name.c',
-  'object-store.c',
   'object.c',
+  'odb.c',
   'oid-array.c',
   'oidmap.c',
   'oidset.c',