/// <summary>
 /// Initializes a new instance of <see cref="CertificateValidationCache"/>.
 /// </summary>
 /// <param name="options">An accessor to <see cref="CertificateValidationCacheOptions"/></param>
 public CertificateValidationCache(IOptions <CertificateValidationCacheOptions> options)
 {
     _options = options.Value;
     _cache   = new MemoryCache(new MemoryCacheOptions {
         SizeLimit = _options.CacheSize
     });
 }
 internal CertificateValidationCache(IOptions <CertificateValidationCacheOptions> options, ISystemClock clock)
 {
     _options = options.Value;
     _cache   = new MemoryCache(new MemoryCacheOptions {
         SizeLimit = _options.CacheSize, Clock = new CachingClock(clock)
     });
     _clock = clock;
 }