[MINOR] stick_table: add support for variable-sized data

[MINOR] stick_table: add support for variable-sized data

Right now we're only able to store a server ID in a sticky session.
The goal is to be able to store anything whose size is known at startup
time. For this, we store the extra data before the stksess pointer,
using a negative offset. It will then be easy to cumulate multiple
data provided they each have their own offset.