Refactored connection pool (#913)

* Refactored ConnectionPool
- Created BaseConnectionPool class
- Created CloudConnectionPool
- connection pool updates are immutable
- resurrect now happens inside getConnection()

* Rewritten connection pool(s) type definitions

* Updated test

* Fixed test

* Fix if check

* Removed old files

* Improve code coverage

* Updated license header

* Fix if check

* Improve code coverage

* Updated coverage script

lib/pool/index.js

@@ -0,0 +1,15 @@
+// Licensed to Elasticsearch B.V under one or more agreements.
+// Elasticsearch B.V licenses this file to you under the Apache 2.0 License.
+// See the LICENSE file in the project root for more information
+
+'use strict'
+
+const BaseConnectionPool = require('./BaseConnectionPool')
+const ConnectionPool = require('./ConnectionPool')
+const CloudConnectionPool = require('./CloudConnectionPool')
+
+module.exports = {
+  BaseConnectionPool,
+  ConnectionPool,
+  CloudConnectionPool
+}