mget REST API should support source parameter

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

This is how to reproduce it:

```sh
curl -XDELETE "http://localhost:9200/test"

curl -XPOST "http://localhost:9200/test/type/1?refresh" -d'{
    "foo": "bar"
}'

curl -XPOST "http://localhost:9200/test/type/_mget" -d'{
    "ids": ["1"]
}'

curl -XGET "http://localhost:9200/test/type/_mget?source=%7B%22ids%22%3A%20%5B%221%22%5D%7D"
```

Closes #4892.
Revert mget yaml test changes

Relative to #4892
Revert mget yaml test changes

Relative to #4892
Bulk: Failed preparsing does not fail whole bulk request

If a preparsing of the source is needed (due to mapping configuration,
which extracts the routing/id value from the source) and the source is not
valid JSON, then the whole bulk request is failed instead of a single
BulkRequest.

This commit ensures, that a broken JSON request is not forwarded to the
destination shard and creates an appropriate BulkItemResponse, which
includes a failure.

This also implied changing the BulkItemResponse serialization, because one
cannot be sure anymore, if a response includes an ID, in case it was not
specified and could not be extracted from the JSON.

Closes #4745