call cb_thread_self instead of pthread_self

Change-Id: I2509b9710b0caa0324ec39a18cf82cd8cb1751b8
Reviewed-on: http://review.couchbase.org/30092
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>
Tested-by: Trond Norbye <trond.norbye@gmail.com>