Add the ability to merge two UnsignedLongSets. This is useful when
we are using ImmutableUnsignedLongSet as a data interchange format
to communicate changes to a MutableUnsignedLongSet.
JIRA: CONTROLLER-2015
Change-Id: Ia84474b685872586722914f20db8f96a3c172f97 Signed-off-by: Robert Varga <robert.varga@pantheon.tech>