Rename MAX_BUFFER_SIZE

MAX_BUFFER_SIZE, which is used for limiting the amount of the data
read from a socket renamed to MXS_MAX_NW_READ_BUFFER_SIZE and moved
from gw.h to limits.h.

Consider removing altogether. Difficult to justify since non-blocking
reads are used and the amount of available data is known.

server/modules/routing/avro/avrorouter.h

@@ -108,7 +108,7 @@ typedef enum avro_binlog_end
 #define TABLE_MAP_MAX_NAME_LEN 64
 
 /** How many bytes each thread tries to send */
-#define AVRO_DATA_BURST_SIZE MAX_BUFFER_SIZE
+#define AVRO_DATA_BURST_SIZE (32 * 1024)
 
 /** A CREATE TABLE abstraction */
 typedef struct table_create