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.

include/jemalloc/internal/prof_structs.h

@@ -167,9 +167,6 @@ struct prof_tdata_s {
 	 */
 	ckh_t			bt2tctx;
 
-	/* Sampling state. */
-	uint64_t		prng_state;
-
 	/* State used to avoid dumping while operating on prof internals. */
 	bool			enq;
 	bool			enq_idump;