Adding the option for configuring the batch size.

[mansoor-sajjad]

👋
We are using pelias-csv-importer to import the huge dataset into elasticsearch
We have experienced that the performance is not optimal with the current batch size, given our resources.

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Here's the reason for this change 🚀

In the process of improving the performance of import, we have experienced that there is no way to configure the batch size when making the bulk import request to elasticsearch via pelias-dbclient.
The pelias-csv-importer is using the pelias-dbclient which has the batch size of 500 hardcoded in the Batch.js.
It would be nice to make it configurable.

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Here's what actually got changed 👏

Added the batchSize configuration option under dbclient configuration, and defaults it to 500 as it is in the pelias-dbclient now.

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After this PR get merged

I have created the PR#125 on pelias-dbclient to use this configuration option. I need to update that PR with the latest version of pelias-config.
I also need to make a PR on the pelias-csv-importer to update the versions of pelias-config and pelias-dbclient to use this configuration option.

[mansoor-sajjad]

Hi @missinglink,

Do you have the opportunity to review this PR, it's been there for a while?