[TIMOB-11671] Clear out unrequired variables at end of run

This PR does a few things.
1. removes the static client objects. Now each instance of TiHttpClient has its own client.
2. nulls out request,response,handler,client and clientThread objects before firing dispose handle event.

I could not reproduce the memory leak as reported in the ticket, though I did see the memory grow which is probably due to XML processing. This is more of a clean up PR to ensure there are no clients in memory when instances of TiHttpClient are GCed.

[JIRA TICKET](https://jira.appcelerator.org/browse/TIMOB-11671)