Refactor and optimize prof sampling initialization.

Makes the prof sample prng use the tsd prng_state.  This allows us to properly
initialize the sample interval event, without having to create tdata.  As a
result, tdata will be created on demand (when a thread reaches the sample
interval bytes allocated), instead of on the first allocation.

src/tsd.c

@@ -233,6 +233,7 @@ tsd_data_init(tsd_t *tsd) {
 	*tsd_prng_statep_get(tsd) = config_debug ? 0 :
 	    (uint64_t)(uintptr_t)tsd;
 
+	/* event_init may use the prng state above. */
 	tsd_thread_event_init(tsd);
 
 	return tsd_tcache_enabled_data_init(tsd);