speeding up create_raw_dataset.py

[ljj7975]

create_raw_dataset.py takes quite a long time to generate datasets.

I thinking multi-threading AudioDatasetMetadataWriter write will do the job.

Also, this process terminates with segfault

[ljj7975]

segfault was happening due to numba
numba/numba#4323

[ljj7975]

Spent some time applying one of the multiprocessing package but the results weren't that good
please refer to https://github.com/castorini/howl/tree/multi_processing_test

[ljj7975]

When writing a dataset, process function should also take in sample (AudioClipExample) and use sample.audio_data when metadata.path does not exist (https://github.com/castorini/howl/blob/master/howl/data/dataset/serialize.py#L67-L72)

[ColonelThirtyTwo]

A simple way to speed this up is to call out to ffmpeg in AudioDatasetWriter rather than doing the conversions in Python (which is slow).