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/aof.c

@@ -442,6 +442,7 @@ struct redisClient *createFakeClient(void) {
 
     selectDb(c,0);
     c->fd = -1;
+    c->name = NULL;
     c->querybuf = sdsempty();
     c->querybuf_peak = 0;
     c->argc = 0;