breacharbiter: avoid infinite loop in exactRetribution

After a shutdown has been initiated, both registrations
for spend ntfns and publishing txns can fail. The current
behavior in the face of such failures is to continue trying,
which is fine if we are online. However, this causes an
infinite loop during shutdown, and lnd cannot exit since
the routine is tracked by the brar's waitgroup.

A simple fix is to select on the brar's quit channel after
detecting a failure from either, allowing the breach arbiter
to break out of this death cycle.

breacharbiter.go

@@ -519,7 +519,16 @@ secondLevelCheck:
 				brarLog.Errorf("unable to check for spentness "+
 					"of out_point=%v: %v",
 					breachedOutput.outpoint, err)
-				continue
+
+				// Registration may have failed if we've been
+				// instructed to shutdown. If so, return here to
+				// avoid entering an infinite loop.
+				select {
+				case <-b.quit:
+					return
+				default:
+					continue
+				}
 			}
 
 			select {
@@ -575,7 +584,15 @@ secondLevelCheck:
 			brarLog.Infof("Attempting to transfer HTLC revocations " +
 				"to the second level")
 			finalTx = nil
-			goto secondLevelCheck
+
+			// Txn publication may fail if we're shutting down.
+			// If so, return to avoid entering an infinite loop.
+			select {
+			case <-b.quit:
+				return
+			default:
+				goto secondLevelCheck
+			}
 		}
 	}