Use UnsignedLongSet instead of RangeSet in metadata

Use UnsignedLongSet instead of RangeSet in metadata

Split UnsignedLongSet into two implementations, mutable and immutable.
Use the mutable implementation in runtime tracking of identifiers and
use the immutable implementation for tracking identifiers in metadata.

The serialization format is kept compatible with RangeSets, although
that implies a minor penalty in serdes.

This switch ends up potentially using more objects for small sets, but
that is offset by not having Cut indirections and most notably being
resistent to allocation of huge arrays.

JIRA: CONTROLLER-2011
Change-Id: I0c84ffaaa4ce39299cef9006784b8aff78dd0f83
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>