VDB-327 Constantinople prep (#135)

* Bump geth to 1.8.20 for Constantinople

* Fix conflicting import/toml source for logrus

vendor/github.com/allegro/bigcache/clock.go

@@ -0,0 +1,14 @@
+package bigcache
+
+import "time"
+
+type clock interface {
+	epoch() int64
+}
+
+type systemClock struct {
+}
+
+func (c systemClock) epoch() int64 {
+	return time.Now().Unix()
+}