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._

test/unit/connection-pool.test.js

@@ -4,6 +4,7 @@ const { test } = require('tap')
 const { URL } = require('url')
 const ConnectionPool = require('../../lib/ConnectionPool')
 const Connection = require('../../lib/Connection')
+const { defaultNodeFilter, roundRobinSelector } = require('../../lib/Transport').internals
 const { connection: { MockConnection, MockConnectionTimeout } } = require('../utils')
 
 test('API', t => {
@@ -227,22 +228,6 @@ test('API', t => {
       pool.getConnection({ filter })
     })
 
-    t.test('filter as ConnectionPool option', t => {
-      t.plan(3)
-
-      const href1 = 'http://localhost:9200/'
-      const href2 = 'http://localhost:9200/other'
-      const pool = new ConnectionPool({
-        Connection,
-        nodeFilter: node => {
-          t.ok('called')
-          return true
-        }
-      })
-      pool.addConnection([href1, href2])
-      t.strictEqual(pool.getConnection().id, href1)
-    })
-
     t.end()
   })
 
@@ -498,27 +483,16 @@ test('API', t => {
 test('Node selector', t => {
   t.test('round-robin', t => {
     t.plan(1)
-    const pool = new ConnectionPool({ Connection, nodeSelector: 'round-robin' })
+    const pool = new ConnectionPool({ Connection })
     pool.addConnection('http://localhost:9200/')
-    t.true(pool.getConnection() instanceof Connection)
+    t.true(pool.getConnection({ selector: roundRobinSelector() }) instanceof Connection)
   })
 
   t.test('random', t => {
     t.plan(1)
-    const pool = new ConnectionPool({ Connection, nodeSelector: 'random' })
+    const pool = new ConnectionPool({ Connection })
     pool.addConnection('http://localhost:9200/')
-    t.true(pool.getConnection() instanceof Connection)
-  })
-
-  t.test('custom function', t => {
-    t.plan(2)
-    const nodeSelector = connections => {
-      t.ok('called')
-      return connections[0]
-    }
-    const pool = new ConnectionPool({ Connection, nodeSelector })
-    pool.addConnection('http://localhost:9200/')
-    t.true(pool.getConnection() instanceof Connection)
+    t.true(pool.getConnection({ selector: roundRobinSelector() }) instanceof Connection)
   })
 
   t.end()
@@ -529,7 +503,7 @@ test('Node filter', t => {
     t.plan(1)
     const pool = new ConnectionPool({ Connection })
     pool.addConnection({ url: new URL('http://localhost:9200/') })
-    t.true(pool.getConnection() instanceof Connection)
+    t.true(pool.getConnection({ filter: defaultNodeFilter }) instanceof Connection)
   })
 
   t.test('Should filter master only nodes', t => {
@@ -544,29 +518,7 @@ test('Node filter', t => {
         ml: false
       }
     })
-    t.strictEqual(pool.getConnection(), null)
-  })
-
-  t.test('custom', t => {
-    t.plan(2)
-    const nodeFilter = node => {
-      t.ok('called')
-      return true
-    }
-    const pool = new ConnectionPool({ Connection, nodeFilter })
-    pool.addConnection({ url: new URL('http://localhost:9200/') })
-    t.true(pool.getConnection() instanceof Connection)
-  })
-
-  t.test('custom (filter)', t => {
-    t.plan(2)
-    const nodeFilter = node => {
-      t.ok('called')
-      return false
-    }
-    const pool = new ConnectionPool({ Connection, nodeFilter })
-    pool.addConnection({ url: new URL('http://localhost:9200/') })
-    t.strictEqual(pool.getConnection(), null)
+    t.strictEqual(pool.getConnection({ filter: defaultNodeFilter }), null)
   })
 
   t.end()