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/Transport.d.ts

@@ -2,7 +2,7 @@
 // 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
 
-import ConnectionPool from './ConnectionPool';
+import { ConnectionPool, CloudConnectionPool } from './pool';
 import Connection from './Connection';
 import Serializer from './Serializer';
 
@@ -23,7 +23,7 @@ declare type emitFn = (event: string | symbol, ...args: any[]) => boolean;
 
 interface TransportOptions {
   emit: emitFn & noopFn;
-  connectionPool: ConnectionPool;
+  connectionPool: ConnectionPool | CloudConnectionPool;
   serializer: Serializer;
   maxRetries: number;
   requestTimeout: number | string;
@@ -113,7 +113,7 @@ export default class Transport {
     DEFAULT: string;
   };
   emit: emitFn & noopFn;
-  connectionPool: ConnectionPool;
+  connectionPool: ConnectionPool | CloudConnectionPool;
   serializer: Serializer;
   maxRetries: number;
   requestTimeout: number;