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

test/unit/cloud-connection-pool.test.js

@@ -0,0 +1,33 @@
+// 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 { test } = require('tap')
+const { CloudConnectionPool } = require('../../lib/pool')
+const Connection = require('../../lib/Connection')
+
+test('Should expose a cloudConnection property', t => {
+  const pool = new CloudConnectionPool({ Connection })
+  pool.addConnection('http://localhost:9200/')
+  t.ok(pool.cloudConnection instanceof Connection)
+  t.end()
+})
+
+test('Get connection should always return cloudConnection', t => {
+  const pool = new CloudConnectionPool({ Connection })
+  const conn = pool.addConnection('http://localhost:9200/')
+  t.deepEqual(pool.getConnection(), conn)
+  t.end()
+})
+
+test('pool.empty should reset cloudConnection', t => {
+  const pool = new CloudConnectionPool({ Connection })
+  pool.addConnection('http://localhost:9200/')
+  t.ok(pool.cloudConnection instanceof Connection)
+  pool.empty(() => {
+    t.strictEqual(pool.cloudConnection, null)
+    t.end()
+  })
+})