Degrade DataNodeContainer.getChildNodes() from Set to Collection

Degrade DataNodeContainer.getChildNodes() from Set to Collection

Datastore-geared workload shows that around 1% of CPU is being used by
AbstractDocumentedDataNodeContainer.getChildNodes(). Of that, around 72%
is spent in ImmutableSet.copyOf(). We could perform that copy in the
constructor, but we can actually do better.

As it turns out, there is already a childNodes map, which is immutable
and indexed via QName present. That indexing guarantees that the values
compare as distinct, so we can reuse childNodes.values(), except for the
contract.

None of the callers care if the return is a Collection, so downgrade the
return, such that we can elide copying the stuff completely. Also fixup
all the callers. Requires https://git.opendaylight.org/gerrit/#/c/9340/
to fix controller-based users.

Change-Id: I310b351d80955a37f6672064dd238bb9cb1e16b1
Signed-off-by: Robert Varga <rovarga@cisco.com>