Add UnsignedLongBitmap

Add UnsignedLongBitmap

Rather than using a ImmutableMap<UnsignedLong, Boolean>, we can do
have a much denser representation with a specialized class. Here we
are introducing an explicit UnsignedLongBitmap, which stores an array
of longs and an array of booleans, ditching the intermediate objects.

Also clean up error reporting, throwing an IOException instead of a
VerifyException when things should go south unexpectedly.

JIRA: CONTROLLER-2013
Change-Id: Ie64da0af68ea2898dc77368afd4fce8abd2cccea
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>