Allow builders to optionally use LinkedHashMap

Allow builders to optionally use LinkedHashMap

Some devices are unreasonably touchy about the order of children
we send them -- i.e. they expect to encounter direct container
children before seeing any augmented children. While we push them
to NormalizedNode builders in this order, the builders are using
plain HashMaps (and rightfully so) to track them, which means
they can get reordered.

This patch introduces a new system property,
org.opendaylight.yangtools.yang.data.impl.schema.builder.retain-child-order,
which when set to "true" will cause DataContainerBuilders to use
LinkedHashMaps to track children, sacrificing efficiency to retain
the original insertion order.

JIRA: YANGTOOLS-984
Change-Id: Ica1616c3fa93559c6458eba4030a5ceaac7be58a
Signed-off-by: Jakub Toth <jtoth@luminanetworks.com>
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>