Bugfix: Pass docvalue_fields for elasticsearch

[idastambuk]

What this PR does / why we need it:
We had a bug reported in #388 (comment) where logs queries to ElasticSearch v7.8.1 with OpenSearch plugin were returning No Data. The reason for this was that we're sending "fields" here to OpenSearch with the timestamp format we desire.
ElasticSearch api, however, expects docvalue_fields instead, so it returns no data. Older versions of Elastic expect fields as well.

There was also some refactoring, since without it, the time field was in some cases added twice in fields or docvalue_fields (e.g. docvalue_fields: ["@timestamp", {"field": "@timestamp", "format": "date_time"}]

• Moved adding the timeField to AddTimeFieldWithStandardizedFormat, where the field, along with its formatting will be added according to the flavor and version. Still not sure if fielddata_fields is completely necessary, but since it's only for Elastic < 5 and logs queries, it might not be relevant in time if we decide to deprecate. The same goes for the new version of AddDocValueFields function
• removed script_fields field since it doesn't seem to be used anywhere

Which issue(s) this PR fixes:

Fixes #405

Special notes for your reviewer:
To test with Elastic 7.8.1:

• run make devenv sources=elasticstack elastic_version=7.8. and make run in main Grafana
• open http://localhost:5601/app/kibana#/home/tutorial_directory/logging and add some sample data (Sample web logs)
• add a new OpenS datasource with this config

• make a logs query/raw document query
• there should be data shown

[kevinwcyu]

LGTM. Do you know why it worked in the frontend, but not the backend path? I didn't see any of these fields in the frontend code.

[idastambuk]

LGTM. Do you know why it worked in the frontend, but not the backend path? I didn't see any of these fields in the frontend code.

I think it's cause the desired format wasn't being set in the query so OpenSearch returns actual data. This however didn't guarantee we will get the expected timestamp format, which caused bugs, so adding it was definitely an improvement.