applier: store instance_id in struct applier

Applier is going to need its numeric ID in order to tell the
future Raft module who is a sender of a Raft message. An
alternative would be to add sender ID to each Raft message, but
this looks like a crutch. Moreover, applier still needs to know
its numeric ID in order to notify Raft about heartbeats from the
peer node.

Needed for #1146

src/box/applier.cc

@@ -1118,6 +1118,7 @@ applier_subscribe(struct applier *applier)
 		vclock_create(&applier->remote_vclock_at_subscribe);
 		xrow_decode_subscribe_response_xc(&row, &cluster_id,
 					&applier->remote_vclock_at_subscribe);
+		applier->instance_id = row.replica_id;
 		/*
 		 * If master didn't send us its cluster id
 		 * assume that it has done all the checks.

src/box/applier.h

@@ -95,6 +95,8 @@ struct applier {
 	ev_tstamp lag;
 	/** The last box_error_code() logged to avoid log flooding */
 	uint32_t last_logged_errcode;
+	/** Remote instance ID. */
+	uint32_t instance_id;
 	/** Remote instance UUID */
 	struct tt_uuid uuid;
 	/** Remote URI (string) */