Do not use ListenableFuture<Void>, which promotes propagation of nulls
across the system. Use instead yang.common.Empty, which has a non-null
singleton value. Also allow commit() to propagate more information
through CommitInfo.
Change-Id: Ib3874c2c84cadcf1f5fb386a38ae5d0a2cb796be Signed-off-by: Robert Varga <robert.varga@pantheon.tech>