MINOR: spoe-server: rename "worker" functions

"worker" name is a little bit generic and it is used in many
places, so it is hard to find the expected symbol.
diff --git a/contrib/spoa_server/spoa.c b/contrib/spoa_server/spoa.c
index 94be6c0..b08c1d6 100644
--- a/contrib/spoa_server/spoa.c
+++ b/contrib/spoa_server/spoa.c
@@ -918,7 +918,7 @@
 }
 
 static void *
-worker(void *data)
+spoa_worker(void *data)
 {
 	struct worker w;
 	struct sockaddr_in client;
@@ -1056,7 +1056,7 @@
 
 		info[0] = sock;
 		info[1] = i+1;
-		if (pthread_create(&ts[i], NULL,  worker, info) < 0) {
+		if (pthread_create(&ts[i], NULL,  spoa_worker, info) < 0) {
 			fprintf(stderr, "Failed to create thread %d: %m\n", i+1);
 			goto error;
 		}