cmd, core, eth, light, trie: add trie read caching layer

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

@@ -0,0 +1,30 @@
+package bigcache
+
+import (
+	"log"
+	"os"
+)
+
+// Logger is invoked when `Config.Verbose=true`
+type Logger interface {
+	Printf(format string, v ...interface{})
+}
+
+// this is a safeguard, breaking on compile time in case
+// `log.Logger` does not adhere to our `Logger` interface.
+// see https://golang.org/doc/faq#guarantee_satisfies_interface
+var _ Logger = &log.Logger{}
+
+// DefaultLogger returns a `Logger` implementation
+// backed by stdlib's log
+func DefaultLogger() *log.Logger {
+	return log.New(os.Stdout, "", log.LstdFlags)
+}
+
+func newLogger(custom Logger) Logger {
+	if custom != nil {
+		return custom
+	}
+
+	return DefaultLogger()
+}