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/connection.test.js

@@ -733,7 +733,7 @@ test('Util.inspect Connection class should hide agent, ssl and auth', t => {
   resurrectTimeout: 0,
   _openRequests: 0,
   status: 'alive',
-  roles: { master: true, data: true, ingest: true, ml: false } }`)
+  roles: { master: true, data: true, ingest: true, ml: false }}`)
   )
 })