Track average message size

[jwp]

In order to properly calibrate fetch sizes and recvsize, some information about what's happening on the wire would be useful. For instance, if we find that a cursor's rows are relatively small on average, we should increase fetchsize in order to reduce the number of syscalls being issued.

The rather low recvsize has proven to be an impediment to performance.

[jwp]

Doing this with the existing the implementation seems like it might be too much work.

[jwp]

A better solution may be to just increase the default recvsize.