Optimizes the index queries to not block the NodeJS event loop (#75716) (#75941)

## Summary

Before this PR you can see event loop block times of:

```ts
formatIndexFields: 7986.884ms
```

After this PR you will see event loop block times of:

```ts
formatIndexFields: 85.012ms
```

within the file:

```ts
x-pack/plugins/security_solution/server/lib/index_fields/elasticsearch_adapter.ts
```

For the GraphQL query of `SourceQuery`/`IndexFields`

This also fixes the issue of `unknown` being returned to the front end by removing code that is no longer functioning as it was intended. Ensure during testing of this PR that blank/default and non exist indexes within `securitySolution:defaultIndex` still work as expected.

Before, notice the `unknown` instead of the `filebeat-*`:
<img width="733" alt="Screen Shot 2020-08-20 at 4 55 52 PM" src="https://user-images.githubusercontent.com/1151048/90949129-f5047900-e402-11ea-9278-b4c7bf5cd16d.png">

After:
<img width="830" alt="Screen Shot 2020-08-20 at 4 56 03 PM" src="https://user-images.githubusercontent.com/1151048/90949133-02b9fe80-e403-11ea-8504-f5bbe043048a.png">

An explanation of how to see the block times for before and after
---

For perf testing you first add timed testing to the file:
```ts
x-pack/plugins/security_solution/server/lib/index_fields/elasticsearch_adapter.ts
```

Before this PR, around lines 42:
```ts
console.time('formatIndexFields'); // <--- start timer
const fields = formatIndexFields(
  responsesIndexFields,
  Object.keys(indexesAliasIndices) as IndexAlias[]
);
console.timeEnd('formatIndexFields'); // <--- outputs the end timer
return fields;
```

After this PR, around lines 42:

```ts
console.time('formatIndexFields'); // <--- start timer
const fields = await formatIndexFields(responsesIndexFields, indices);
console.timeEnd('formatIndexFields');  // <--- outputs the end timer
return fields;
```

And then reload the security solutions application web page here:
```
http://localhost:5601/app/security/timelines/default
```

Be sure to load it _twice_ for testing as NodeJS will sometimes report better numbers the second time as it does optimizations after the first time it encounters some code paths.

You will begin to see numbers similar to this before this PR:

```ts
formatIndexFields: 2553.279ms
```

This indicates that it is blocking the event loop for ~2.5 seconds befofe this fix. If you add additional indexes to your `securitySolution:defaultIndex` indexes that have additional fields then this amount will increase exponentially. For developers using our test servers I created two other indexes called delme-1 and delme-2 with additional mappings you can add like below

```ts
apm-*-transaction*, auditbeat-*, endgame-*, filebeat-*, logs-*, packetbeat-*, winlogbeat-*, delme-1, delme-2
```

<img width="980" alt="Screen Shot 2020-08-21 at 8 21 50 PM" src="https://user-images.githubusercontent.com/1151048/90949142-211ffa00-e403-11ea-8ab2-f66de977dce3.png">

Then you are going to see times approaching 8 seconds of blocking the event loop like so:

```ts
formatIndexFields: 7986.884ms
```

After this fix on the first pass unoptimized it will report

```ts
formatIndexFields: 373.082ms
```

Then after it optimizes the code paths on a second page load it will report

```ts
formatIndexFields: 84.304ms
```

### Checklist

Delete any items that are not applicable to this PR.

- [x] [Unit or functional tests](https://www.elastic.co/guide/en/kibana/master/development-tests.html) were updated or added to match the most common scenarios

Author: FrankHassanabad