Fix spurious 'Socket receive buffer full' errors

[jamadden]

(This may be related to #34.)

In some cases, it was possible for the buffer to be reported as full when in fact it wasn't; it had lots of space at the beginning of the buffer, but the write cursor reached the end. The skip method never detected the read and write pointers aligning and so never reset the buffer. This would happen when reading a large result set from the server very quickly; I suspect its occurrence depends on factors like network connection bandwidth and latency as well as the net_buffer_length of the server.

This builds on my previous PR.