Client types generalized.

Because of output buffer limits Redis internals had this idea of type of
clients: normal, pubsub, slave. It is possible to set different output
buffer limits for the three kinds of clients.

However all the macros and API were named after output buffer limit
classes, while the idea of a client type is a generic one that can be
reused.

This commit does two things:

1) Rename the API and defines with more general names.
2) Change the class of clients executing the MONITOR command from "slave"
   to "normal".

"2" is a good idea because you want to have very special settings for
slaves, that are not a good idea for MONITOR clients that are instead
normal clients even if they are conceptually slave-alike (since it is a
push protocol).

The backward-compatibility breakage resulting from "2" is considered to
be minimal to care, since MONITOR is a debugging command, and because
anyway this change is not going to break the format or the behavior, but
just when a connection is closed on big output buffer issues.

src/redis.c

@@ -1485,7 +1485,7 @@ void initServerConfig() {
     server.repl_no_slaves_since = time(NULL);
 
     /* Client output buffer limits */
-    for (j = 0; j < REDIS_CLIENT_LIMIT_NUM_CLASSES; j++)
+    for (j = 0; j < REDIS_CLIENT_TYPE_COUNT; j++)
         server.client_obuf_limits[j] = clientBufferLimitsDefaults[j];
 
     /* Double constants initialization */