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/Connection.d.ts

@@ -6,7 +6,7 @@
 
 import { URL } from 'url';
 import { inspect, InspectOptions } from 'util';
-import { ApiKeyAuth, BasicAuth } from './ConnectionPool'
+import { ApiKeyAuth, BasicAuth } from './pool'
 import * as http from 'http';
 import { ConnectionOptions as TlsConnectionOptions } from 'tls';