getroute: change definition (and pay default) for riskfactor.

Up until now, riskfactor was useless due to implementation bugs, and
also the default setting is wrong (too low to have an effect on
reasonable payment scenarios).

Let's simplify the definition (by assuming that P(failure) of a node
is 1), to make it a simple percentage.  I examined the current network
fees to see what would work, and under this definition, a default of
10 seems reasonable (equivalent to 1000 under the old definition).

It is *this* change which finally fixes our test case!  The riskfactor
is now 40msat (1500000 * 14 * 10 / 5259600 = 39.9), comparable with
worst-case fuzz is 50msat (1001 * 0.05 = 50).

Signed-off-by: Rusty Russell <rusty@rustcorp.com.au>

tests/test_pay.py

@@ -1341,7 +1341,6 @@ def test_pay_routeboost(node_factory, bitcoind):
         assert [h['channel'] for h in attempts[2]['routehint']] == [r['short_channel_id'] for r in routel3l5]
 
 
-@pytest.mark.xfail(strict=True)
 def test_pay_direct(node_factory, bitcoind):
     """Check that we prefer the direct route.
     """