bulk insert batch requests

[michaelcheah]

What this PR does / why we need it:
The previous implementation does a for loop and inserts each item in a batch request as an individual update to elasticsearch. This PR changes inserting batch requests by using bulk updates. This provides a significant improvement in performance.

Special notes for your reviewer:

• The content of docs created in elasticsearch is unchanged.
• One breaking change introduced where the response of for the request logger is now empty. i.e. previously a post request to the request logger would return a response body with the following content (this is the elasticsearch API response)

  [{"_id": "abc123", "_index": "inference-log-seldon-seldon-iris-default", "_primary_term": 1, 
  "_seq_no": 2, "_shards": {"failed": 0, "successful": 1, "total": 2}, "_type": "_doc", 
  "_version": 1, "forced_refresh": true, "result": "created"}]]

  However, now an empty list is returned. It is possible to return a similar looking response by constructing it, but the Python elasticsearch client for bulk updates does not provide any way to return the same content.

Does this PR introduce a user-facing change?:

seldon-request-logger post response when receiving a request with batch data does not include elasticsearch API response - returns an empty list

[seldondev]

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[SachinVarghese]

print the item_request_id that yielded

[SachinVarghese]

is this redundant?

[SachinVarghese]

use helper function log_helper.DOC_TYPE_NAME