rename "alternate_object_database" to "object_directory"

rename "alternate_object_database" to "object_directory"

In preparation for unifying the handling of alt odb's and the normal
repo object directory, let's use a more neutral name. This patch is
purely mechanical, swapping the type name, and converting any variables
named "alt" to "odb". There should be no functional change, but it will
reduce the noise in subsequent diffs.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>