Rename StreamlineEventImpl to DefaultStreamlineEvent and move it to streamline-sdk

[priyank5485]

As per the discussion here https://github.com/hortonworks/streamline/pull/963#discussion_r144694050

[hmcl]

I suggest that the interface is called Event or InternalEvent (if you want to avoid possible name collision with some libraries) and the default (for streamline) implementation is called StreamlineEvent. DefaultStreamlineEvent is too long of a name and I don't think the naming prefix default is appropriate here.

[arunmahadevan]

The interface is already called StreamlineEvent. Lets retain that else will lead to huge refactoring and maybe backward incompatible. For our internal implementation DefaultStreamlineEvent or StreamlineEventInternal sounds good.

We also need to re-visit if the interface should extend the map interface. It better to get this done in the next release before its too late.

[hmcl]

@arunmahadevan if possible I would like to avoid such long names. I understand the concern on changing the name of the interface. Perhaps we can call the default implementation simply InternalEvent or DefaultEvent as the substring Streamline can become quite verbose imho.