Fix findbugs violations in md-sal - part 3

Fix findbugs violations in md-sal - part 3

- sal-binding-broker
- sal-connector-api
- sal-dummy-distributed-datastore
- messagebus-impl
- mdsal-trace-dom-impl

- May expose internal representation by returning reference to mutable object
- May expose internal representation by incorporating reference to mutable object
- Private method is never called
- Non-transient non-serializable instance field in serializable class
- Class is Serializable, but doesn't define serialVersionUID
- Parameter must be non-null but is marked as nullable
- Unread field
- Consider returning a zero length array rather than null
- Useless object created
- Method ignores return value
- Incorrect lazy initialization of static field
- Should be a static inner class

Change-Id: Ia8847db80bca98c6f7ff7aae267efc408a5dd8fd
Signed-off-by: Tom Pantelis <tompantelis@gmail.com>