comp_thread_ctxt_t: Remove ctrl_mutex, ctrl_cond, started. We do not
actually need them for anything.

destroy_worker_thread(): Split from destroy_worker_threads().

create_worker_threads(): We already initialize
thd->data_avail=FALSE and thd->cancelled=FALSE before
invoking pthread_create(). If any thread creation fails,
clean up by destroy_worker_thread().

compress_worker_thread_func(): Assume that thd->started and
thd->data_avail are already initialized.

Reviewed by: Vladislav Vaintroub