Add centralized IP/Peer formatting functions

This stops us from needing to manually check against ":" to
add brackets around IPv6 addresses everywhere.

src/sds.h

@@ -91,6 +91,7 @@ sds sdscatrepr(sds s, const char *p, size_t len);
 sds *sdssplitargs(const char *line, int *argc);
 sds sdsmapchars(sds s, const char *from, const char *to, size_t setlen);
 sds sdsjoin(char **argv, int argc, char *sep);
+sds sdsformatip(char *ip, int port);
 
 /* Low level functions exposed to the user API */
 sds sdsMakeRoomFor(sds s, size_t addlen);