Limit resultset read size

In prior versions, MaxScale would read resultset data from server-side socket without limit. In practice, the read would end when the socket was empty, which would then allow more data to be sent by the server. This is usually not an issue as the data is then written to the client socket. However, if client connection is a bottleneck, quite a bit of data could end up stored in the client-side write buffer.

The resultset read code could instead check the size of the client-side write queue, and limit the amount of data read such that the write queue cannot significantly overshoot the writeq_high_water-setting.

Other optimizations to related IO may also be added.