Child client support (#768)

With this pr we introduce the `client.child` API, which returns a new client instance that shares the connection pool with the parent client.
This feature can be handy if you need to have multiple client instances with different configurations, but with a shared connection pool.

Example:

```js
const { Client } = require('@elastic/elasticsearch')
const client = new Client({ node: 'http://localhost:9200' })
const child = client.child({
  headers: { 'x-foo': 'bar' },
  requestTimeout: 1000
})

client.info(console.log)
child.info(console.log)
```

**Open questions:**

* Currently, the event emitter is shared between the parent and the child(ren), is it ok?
* Currently, if you extend the parent client, the child client will have the same extensions, while if the child client adds an extension, the parent client will not be extended. Is it ok?

**Caveats:**

* You can override _any_ option except for the connection pool specific options (`ssl`, `agent`, `pingTimeout`, `Connection`, and `resurrectStrategy`).
* You can't specify a new `Connection` class.
* If you call `close` in any of the parent/child clients, every client will be closed.

_Note: the `nodeFilter` and `nodeSelector` options are now `Transport` options and no longer `ConnectionPool` options._

lib/Transport.d.ts

@@ -2,6 +2,14 @@ import ConnectionPool from './ConnectionPool';
 import Connection from './Connection';
 import Serializer from './Serializer';
 
+export interface nodeSelectorFn {
+  (connections: Connection[]): Connection;
+}
+
+export interface nodeFilterFn {
+  (connection: Connection): boolean;
+}
+
 declare type noopFn = (...args: any[]) => void;
 declare type emitFn = (event: string | symbol, ...args: any[]) => boolean;
 
@@ -17,6 +25,9 @@ interface TransportOptions {
   sniffOnConnectionFault: boolean;
   sniffEndpoint: string;
   sniffOnStart: boolean;
+  nodeFilter?: nodeFilterFn;
+  nodeSelector?: string | nodeSelectorFn;
+  headers?: anyObject;
 }
 
 export interface ApiResponse {