tor-keymgr: Rename the primary keystore for clarity.

Previously, arti's primary keystore was referred to as its "default" keystore. However, "default" is inaccurate here: there is no way to meaningfully override this "default" (the "default" store acts as the main keystore). Throughout the codebase, we query all keystores for keys (including the secondary ones), but only ever write to the default/primary keystore. This is OK for now, because it enables us to have one mutable keystore, and multiple secondary, read-only stores.