Do not create worker thread if threading isn't enabled

Currently, we would create worker threads even if number of compression
thread is set to 1. This works, but having context switches and data
exchange between two threads is complete overhead if main thread is just
blocking on the worker thread.

Test: th
Change-Id: I02f98ee1e0c4889dc1ae602eb06667b05796d3f0
3 files changed