Performance optimisations.

Please find in this Pull Request a series of improvements to the handling of the chunked packet protocol implementation of qmynd. Simple testing made with the sakila database and pgloader, as in http://tapoueh.org/blog/2013/11/12-migrating-sakila, shows the new code to be twice as fast as the current one, when disabling compression over a local connection.

The series shows a first idea where the payload was still eagerly fetched. Much better performances can be obtained by simply implementing a chunked stream directly as in the final patch.

Please consider merging the new code, it would enable be to use qmynd in pgloader while not paying too high a price for a better model of memory management (one row at a time) when compared to cl-mysql.