new vio functions with unit tests

Hi,

• currently there are no unit tests for vio.
• the implementation of http://my-replication-life.blogspot.se/2014/03/faster-semisync-replication.html memcpy a vio-object and assumes that vio->mysql_socket is pollable.

this works poorly for us that have implemented our own vio backend.
this patch adds new vio functions and adds unit tests for them.