routerrpc+routing: adapt payment session for multi shard send

Modifies the payment session to launch additional pathfinding attempts
for lower amounts. If a single shot payment isn't possible, the goal is
to try to complete the payment using multiple htlcs. In previous
commits, the payment lifecycle has been prepared to deal with
partial-amount routes returned from the payment session. It will query
for additional shards if needed.

Additionally a new rpc payment parameter is added that controls the
maximum number of shards that will be used for the payment.

routing/payment_session_source.go

@@ -86,6 +86,7 @@ func (m *SessionSource) NewPaymentSession(p *LightningPayment) (
 		getRoutingGraph:   m.getRoutingGraph,
 		pathFindingConfig: m.PathFindingConfig,
 		missionControl:    m.MissionControl,
+		minShardAmt:       DefaultShardMinAmt,
 	}, nil
 }