Inline Entry.contains()

MutableUnsignedLongSet.add() has half of the check done in contains()
as an invariant. Inline the second part there, improving performance a
bit.

Thisleaves only UnsignedLongSet.contains() as the sole caller of
Entry.contains(). Inline it there, reducing the footprint a bit.

JIRA: CONTROLLER-2014
Change-Id: Ie38283a1fc46aff8fbc717a23aa9a068805636db
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>