func (mc *MemoryCache) LastModified() time.Time {
	return mc.lastModified
}
LastModified returns the timestamp of the latest uploaded Diagnosis Key in the cache.
package diag
import (
	"bytes"
	"io"
	"time"
)
type Cache interface {
	Set(buf []byte, lastModified time.Time) error
	LastModified() time.Time
	ReadSeeker(after [16]byte) io.ReadSeeker
}
type MemoryCache struct {
	buf          []byte
	lastModified time.Time
}
func (mc *MemoryCache) Set(buf []byte, lastModified time.Time) error {
	mc.buf = buf
	mc.lastModified = lastModified
	return nil
}