Suggestions

[sixcorners]

I kind of wish there were fewer checked exceptions.
  Throwing Exception from close is rough.
  The BackendNotSupportedException doesn't seem very actionable.
I wish there was a default public static final instance.
  Kind of like jackson's JsonMapper.shared().
I wish that keeping an unclosed instance of Keyring around wouldn't prevent the app from exiting.
  Is it possible to make any threads it makes daemon threads?
  Does anything bad happen if the Keyring isn't closed?