Move timeutils functions to the only places where they are used.

- Move time json marshaling to the jsonlog package: this is a docker
  internal hack that we should not promote as a library.
- Move Timestamp encoding/decoding functions to the API types: This is
  only used there. It could be a standalone library but I don't this
it's worth having a separated repo for this. It could introduce more
complexity than it solves.

Signed-off-by: David Calavera <david.calavera@gmail.com>

jsonlog/time_marshalling.go

@@ -0,0 +1,27 @@
+// Package jsonlog provides helper functions to parse and print time (time.Time) as JSON.
+package jsonlog
+
+import (
+	"errors"
+	"time"
+)
+
+const (
+	// RFC3339NanoFixed is our own version of RFC339Nano because we want one
+	// that pads the nano seconds part with zeros to ensure
+	// the timestamps are aligned in the logs.
+	RFC3339NanoFixed = "2006-01-02T15:04:05.000000000Z07:00"
+	// JSONFormat is the format used by FastMarshalJSON
+	JSONFormat = `"` + time.RFC3339Nano + `"`
+)
+
+// FastTimeMarshalJSON avoids one of the extra allocations that
+// time.MarshalJSON is making.
+func FastTimeMarshalJSON(t time.Time) (string, error) {
+	if y := t.Year(); y < 0 || y >= 10000 {
+		// RFC 3339 is clear that years are 4 digits exactly.
+		// See golang.org/issue/4556#c15 for more discussion.
+		return "", errors.New("time.MarshalJSON: year outside of range [0,9999]")
+	}
+	return t.Format(JSONFormat), nil
+}