fixed nested example response in docs

Closes #4935
Improve test stability
Added dedicated thread pool cat api, that can show all thread pool related statistic (size, rejected, queue etc.) for all thread pools (get, search, index etc.)
By default active, rejected and queue thread statistics are included for the index, bulk and search thread pool.
Other thread statistics of other thread pools can be included via the `h` query string parameter.

Closes #4907
Remove waiting for green in CopyToMapperIntegrationTests
[TEST] Fixed whitespace in cluster.state/20_filtering.yaml
scroll REST API should support source parameter

As stated in documentation, we should support `?source=` parameter in `/_search/scroll` REST operations.

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"
curl -XPOST "http://localhost:9200/test/type/1" -d'
{
    "foo": "bar"
}'

# This one works
curl -XPOST "http://localhost:9200/_search/scroll" -d "FAKESCROLLID"

# This one gives: {"error":"Failed to derive xcontent from org.elasticsearch.common.bytes.BytesArray@0"}
curl -XGET "http://localhost:9200/_search/scroll/?source=FAKESCROLLID"
```

Closes #4941.
Add throttling to snaphost and restore operations

Closes #4855