Update concepts.Registration

Update concepts.Registration

Do not allow Registration.close() to throw checked exceptions. While it
is a good practice to check for exceptions in foreign code, it is purely
optional and should include RuntimeExceptions as well.

Also update AbstractRegistration to enforce identity hashCode/equals and
also toString() format.

Change-Id: I8d70273e1a42943288a16ac8edf40381707da186
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>