routing: delay initial zombie prune by 30 sec

Since zombie pruning can be very slow on some devices (e.g. mobile) it
would stall lnd startup. Since it is not essential for pruning to be
finished for lnd to be functional, we instead delay the initial prune by
30 seconds.

Note that we could also wait for the graphPruneInterval to tick, but
since this is by default 2 hours, it is unlikely that a mobile app will
ever be open that long.

routing/router_test.go

@@ -1541,6 +1541,9 @@ func TestWakeUpOnStaleBranch(t *testing.T) {
 		Control:            makeMockControlTower(),
 		ChannelPruneExpiry: time.Hour * 24,
 		GraphPruneInterval: time.Hour * 2,
+
+		// We'll set the delay to zero to prune immediately.
+		FirstTimePruneDelay: 0,
 	})
 	if err != nil {
 		t.Fatalf("unable to create router %v", err)
@@ -2157,6 +2160,9 @@ func testPruneChannelGraphDoubleDisabled(t *testing.T, assumeValid bool) {
 	if !assumeValid {
 		assertChannelsPruned(t, ctx.graph, testChannels)
 	} else {
+		// Sleep to allow the pruning to finish.
+		time.Sleep(200 * time.Millisecond)
+
 		prunedChannel := testChannels[len(testChannels)-1].ChannelID
 		assertChannelsPruned(t, ctx.graph, testChannels, prunedChannel)
 	}