Introduced flag for enabling Event Engine

[jguz-pubnub]

1. Removed associated types from connection statuses in order not to introduce more breaking changes
   • What's been added is connectionError case only
2. Added enableEventEngine flag in PubNubConfiguration and ee query parameter
   • This query parameter is added for subscribe, heartbeat, and leave calls assuming that the enableEventEngine flag is set to true
3. SubscriptionSession and SubscribeSessionFactory are being deprecated
   • These classes are marked as public, so customers can subscribe/unsubscribe without using PubNub client. This should be prohibited and obsoleted with a major upgrade
4. Added strategy pattern to switch between old and new implementation for the Subscribe loop
   • This is temporary and should be removed if we decide to get rid of enableEventEngine flag and always use EE
5. Reusing existing unit tests in SubscribeRouterTests to test old & new implementations