object-file: split out functions relating to object store subsystem

object-file: split out functions relating to object store subsystem

While we have the "object-store.h" header, most of the functionality for
object stores is actually hosted in "object-file.c". This makes it hard
to find relevant functions and causes us to mix up concerns.

Split out functions relating to the object store subsystem into a new
"object-store.c" file.

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