CLIENT GETNAME and CLIENT SETNAME introduced.

Sometimes it is much simpler to debug complex Redis installations if it
is possible to assign clients a name that is displayed in the CLIENT
LIST output.

This is the case, for example, for "leaked" connections. The ability to
provide a name to the client makes it quite trivial to understand what
is the part of the code implementing the client not releasing the
resources appropriately.

Behavior:

    CLIENT SETNAME: set a name for the client, or remove the current
                    name if an empty name is set.
    CLIENT GETNAME: get the current name, or a nil.
    CLIENT LIST: now displays the client name if any.

Thanks to Mark Gravell for pushing this idea forward.

src/redis.h

@@ -382,6 +382,7 @@ typedef struct redisClient {
     int fd;
     redisDb *db;
     int dictid;
+    robj *name;             /* As set by CLIENT SETNAME */
     sds querybuf;
     size_t querybuf_peak;   /* Recent (100ms or more) peak of querybuf size */
     int argc;