object-store: rename `raw_object_store` to `object_database`

The `raw_object_store` structure is the central entry point for reading and writing objects in a repository. The main purpose of this structure is to manage object directories and provide an interface to access and write objects in those object directories. Right now, many of the functions associated with the raw object store implicitly rely on `the_repository` to get access to its `objects` pointer, which is the `raw_object_store`. As we want to generally get rid of using `the_repository` across our codebase we will have to convert this implicit dependency on this global variable into an explicit parameter. This conversion can be done by simply passing in an explicit pointer to a repository and then using its `->objects` pointer. But there is a second effort underway, which is to make the object subsystem more selfcontained so that we can eventually have pluggable object backends. As such, passing in a repository wouldn't make a ton of sense, and the goal is to convert the object store interfaces such that we always pass in a reference to the `raw_object_store` instead. This will expose the `raw_object_store` type to a lot more callers though, which surfaces that this type is named somewhat awkwardly. The "raw_" prefix makes readers wonder whether there is a non-raw variant of the object store, but there isn't. Furthermore, we nowadays want to name functions in a way that they can be clearly attributed to a specific subsystem, but calling them e.g. `raw_object_store_has_object()` is just too unwieldy, even when dropping the "raw_" prefix. Instead, rename the structure to `object_database`. This term is already used a lot throughout our codebase, and it cannot easily be mistaken for "object directories", either. Furthermore, its acronym ODB is already well-known and works well as part of a function's name, like for example `odb_has_object()`. Signed-off-by: Patrick Steinhardt <ps@pks.im> Signed-off-by: Junio C Hamano <gitster@pobox.com>