MEDIUM: http_act: define set-timeout server/tunnel action

MEDIUM: http_act: define set-timeout server/tunnel action

Add a new http-request action 'set-timeout [server/tunnel]'. This action
can be used to update the server or tunnel timeout of a stream. It takes
two parameters, the timeout name to update and the new timeout value.
This rule is only valid for a proxy with backend capabilities. The
timeout value cannot be null. A sample expression can also be used
instead of a plain value.