Conditional async_client liftetime_thread creation

Create a thread and add it to io_service only if io_service is internal. If internal - do not change it.
Fix uri swap crash
Fix tabs
Create thread in async_client when io_service is own

Fixes a bug in previews commit when if was inverted.
Merge pull request #431 from eakraly/0.11-devel

Conditional async_client liftetime_thread creation and fix for URI swap bug.