MXS-2259: Limit size of client reads

Given the assumption that queries are rarely 16MB long and that
realistically the only time that happens is during a large dump of data,
we can limit the size of a single read to at most one MariaDB/MySQL packet
at a time. This change allows the network throttling to engage a lot
sooner and reduces the maximum overshoot of throtting to 16MB.

include/maxscale/dcb.h

@@ -265,6 +265,7 @@ DCB* dcb_accept(DCB* listener);
 DCB* dcb_alloc(dcb_role_t, struct servlistener*);
 DCB* dcb_connect(struct server*, struct session*, const char*);
 int  dcb_read(DCB*, GWBUF**, int);
+int  dcb_bytes_readable(DCB* dcb);
 int  dcb_drain_writeq(DCB*);
 void dcb_close(DCB*);