routing: separate MissionControl from MissionControlManager

This commit renames the previous MissionControl to MissionController and
the previous MissionController interface to MissionControlQuerier. This
is done because soon the (new) MissionController will back multiple
namespaced MissionControl instances.  For now, it just houses a single
MissionControl in the default namespace.

This commit also replaces the MissionControl's `now` function with a
`clock.Clock`.

routing/payment_session_source.go

@@ -11,7 +11,7 @@ import (
 	"github.com/lightningnetwork/lnd/zpay32"
 )
 
-// A compile time assertion to ensure MissionControl meets the
+// A compile time assertion to ensure SessionSource meets the
 // PaymentSessionSource interface.
 var _ PaymentSessionSource = (*SessionSource)(nil)
 
@@ -40,7 +40,7 @@ type SessionSource struct {
 	// then take into account this set of pruned vertexes/edges to reduce
 	// route failure and pass on graph information gained to the next
 	// execution.
-	MissionControl MissionController
+	MissionControl MissionControlQuerier
 
 	// PathFindingConfig defines global parameters that control the
 	// trade-off in path finding between fees and probability.