On stack buffer to read replies set to 16k (was 2k).

It was verified experimentally that this value, on Linux kernels, provides
better performances compared to the 2k value. However larger values
apparently don't produce any noticeable effect on performances.

hiredis.c

@@ -1068,7 +1068,7 @@ int redisSetTimeout(redisContext *c, struct timeval tv) {
  * After this function is called, you may use redisContextReadReply to
  * see if there is a reply available. */
 int redisBufferRead(redisContext *c) {
-    char buf[2048];
+    char buf[1024*16];
     int nread;
 
     /* Return early when the context has seen an error. */