Snapshotting start/end notifications

## Notify processor of snapshotting start/end:
```
fn on_source_snapshotting_started(
&mut self,
fw: &mut dyn ProcessorChannelForwarder,
) -> Result<(), ExecutionError>;
fn on_source_snapshotting_done(
&mut self,
fw: &mut dyn ProcessorChannelForwarder,
) -> Result<(), ExecutionError>;
```
*We must pass `ProcessorChannelForwarder` in the processor case, so processor can take action and decide to flush all pending
operations before exiting the method*

## Notify sinks of snapshotting start
```
fn on_source_snapshotting_started(
&mut self
) -> Result<(), ExecutionError>;
```