Streamline API naming for `Server*` / `*Callable`

[th0ger]

I just tried to get an overview of the server classes fingerprints (pynumaflow 0.7.0);

module	class_name	arg_name	arg_type
pynumaflow.mapper	MapServer	mapper_instance	MapSyncCallable
pynumaflow.mapper	MapAsyncServer	mapper_instance	MapAsyncCallable
pynumaflow.mapper	MapMultiprocServer	mapper_instance	MapSyncCallable
pynumaflow.reducer	ReduceAsyncServer	reducer_handler	ReduceCallable
pynumaflow.mapstreamer	MapStreamAsyncServer	map_stream_instance	MapStreamCallable
pynumaflow.sourcetransformer	SourceTransformServer	source_transform_instance	SourceTransformCallable
pynumaflow.sourcetransformer	SourceTransformMultiProcServer	source_transform_instance	SourceTransformCallable
pynumaflow.sourcer	SourceServer	sourcer_instance	SourceCallable
pynumaflow.sourcer	SourceAsyncServer	sourcer_instance	SourceCallable
pynumaflow.sinker	SinkServer	sinker_instance	SyncSinkCallable
pynumaflow.sinker	SinkAsyncServer	sinker_instance	AsyncSinkCallable
pynumaflow.sideinput	SideInputServer	side_input_instance	RetrieverCallable

• reducer_handler stands out, suggesting rename reducer_instance.
• Consider dropping Sync syntax altogether, since it is verbose and will lead to inconsistent syntax over time. For example ReduceCallable needs to be renamed to ReduceSyncCallable when ReduceAsyncCallable is implemented. The Server* class names follows this convention, but the *Callables don't.
• AsyncSinkCallable stands out to MapSyncCallable/MapAsyncCallable/MapSyncCallable in verb ordering, suggesting consitent naming SinkAsyncCallable.

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