Turn Preconditions.checkNotNull into Objects.requireNonNull, use it
as static import. Eliminate the use of Optional/Function/Supplier
concepts, which are now native to Java 8.
Change-Id: Ie470283e1b4f706b0601a191626bd1e036d4d723 Signed-off-by: Robert Varga <robert.varga@pantheon.tech>