pipeline: do not sdsrange querybuf unless all commands processed

This is an optimization for processing pipeline, we discussed a
problem in issue #5229: clients may be paused if we apply `CLIENT
PAUSE` command, and then querybuf may grow too large, the cost of
memmove in sdsrange after parsing a completed command will be
horrible. The optimization is that parsing all commands in queyrbuf
, after that we can just call sdsrange only once.

src/server.h

@@ -710,6 +710,7 @@ typedef struct client {
     redisDb *db;            /* Pointer to currently SELECTed DB. */
     robj *name;             /* As set by CLIENT SETNAME. */
     sds querybuf;           /* Buffer we use to accumulate client queries. */
+    size_t qb_pos;          /* The position we have read in querybuf. */
     sds pending_querybuf;   /* If this client is flagged as master, this buffer
                                represents the yet not applied portion of the
                                replication stream that we are receiving from