Do not instantiate augmentation HashMap in DTO builders

Do not instantiate augmentation HashMap in DTO builders

Tracing has revealed that the OpenFlowPlugin creates a lot of builders,
instantiation a million HashMaps, which accounts for 1% of CPU time.
Most of these maps are never touched, so we can easily optimize this
away by lazily instantiating a HashMap when the first augmentation is
added. Also prevents copying of empty augmentation in the copy
constructor.

Change-Id: I592fe6af65c9f3b65038def8bb4069666bfdc375
Signed-off-by: Robert Varga <rovarga@cisco.com>