Made each source start from its newest stored release, with --since as an override

Without --since, fetcher service and job-search serve start each source
from the newest release stored for it (the new store.LatestReleaseDate,
by Source), so a restart carries on where it left off instead of fetching
the last day again; with nothing stored, the client's default still
applies. --since overrides it for every source. The Find a Tender source's
name is now a constant, as the Source its opportunities are stored with.

The README's guide to adding a source is brought up to date with the
current worker, start and config type names.

internal/store/store_test.go

@@ -837,3 +837,37 @@ func TestWonAndLostAreExclusive(t *testing.T) {
 		}
 	}
 }
+
+// LatestReleaseDate is the newest release of one source's opportunities only,
+// and the zero time for a source with none stored.
+func TestLatestReleaseDate(t *testing.T) {
+	s := openStore(t)
+	ctx := context.Background()
+
+	latest, err := s.LatestReleaseDate(ctx, "FindATender")
+	if err != nil || !latest.IsZero() {
+		t.Fatalf("empty store: %v, %v; want the zero time", latest, err)
+	}
+
+	mustSave(t, s, opportunity("ocds-1", "Older", 1, "r1"))
+	mustSave(t, s, opportunity("ocds-2", "Newer", 5, "r1"))
+	other := opportunity("ocds-3", "Another source's", 9, "r1")
+	other.Source = "FTSMock"
+	mustSave(t, s, other)
+
+	for source, uuid := range map[string]string{
+		"FindATender": "ocds-2",
+		"FTSMock":     "ocds-3",
+	} {
+		want := mustGet(t, s, uuid).LatestReleaseDate
+		latest, err := s.LatestReleaseDate(ctx, source)
+		if err != nil || !latest.Equal(want) {
+			t.Errorf("%s: %v, %v; want %v", source, latest, err, want)
+		}
+	}
+
+	latest, err = s.LatestReleaseDate(ctx, "Nowhere")
+	if err != nil || !latest.IsZero() {
+		t.Errorf("unknown source: %v, %v; want the zero time", latest, err)
+	}
+}