Bugfix: Re-probe longpoll header for each pool alive check, including retries when a preferred protocol fails

Without this, the longpoll header missing on GBT replies will be remembered even for a subsequent getwork.
Re-probing every time also buys us the ability to pick up on changes eventually.

miner.c

@@ -4436,6 +4436,7 @@ tryagain:
 	rpc_req = prepare_rpc_req(work);
 
 	applog(LOG_INFO, "Testing pool %s", pool->rpc_url);
+	pool->probed = false;
 	val = json_rpc_call(curl, pool->rpc_url, pool->rpc_userpass, rpc_req,
 			true, false, &rolltime, pool, false);