[MINOR] stick-tables: add support for "stick on hdr"

[MINOR] stick-tables: add support for "stick on hdr"

It is now possible to stick on an IP address found in a HTTP header. Right
now only the last occurrence of the header can be used, which is generally
enough for most uses. Also, the header extraction rule only knows how to
convert the header to IP. Later it will be usable as a plain string with
an implicit conversion, and the syntax will not change.