sd-journal: introduce journal_file_pin_object()

sd-journal: introduce journal_file_pin_object()

Previously, OBJECT_UNUSED was used for 'pinning' the mmap cache for an
object. But, OBJECT_UNUSED is also used for reading object when type
cannot be determined before read, e.g. when reading the tail object.

Let's introduce another category for pinning mmap cache, and use it when
we want to temporary pin an object.