BUG-4514: clean children in InternalJMXRegistrator

BUG-4514: clean children in InternalJMXRegistrator

Retaining children once they have been closed can lead to a leak, make
sure we call back to parent to remove ourselves from its list.

This includes a refactor to hide InternalJMXRegistrator, which becomes
abstract and has two subclasses: Root and Nested. This allows us to use
proper synchronization between closing of the child and parent
registrators. Also saves a bit of memory.

Also clean up {Module,Transaction}JMXRegistrator constructors to not do
a createChild(), as that fails to cleanup immediate children of the
Root, leading to empty InternalJMXRegistrators being collected in Root's
child list.

Change-Id: I9a4708b67777ca6033e5a83c586b3f78692dff2a
Signed-off-by: Robert Varga <rovarga@cisco.com>