[MEDIUM] appsession: add "len", "prefix" and "mode" options

[MEDIUM] appsession: add "len", "prefix" and "mode" options

To sum up :
- len : it's now the max number of characters for the value, preventing
  garbaged results.
- a new option "prefix" is added, this allows to use dynamic cookie
  names (e.g. ASPSESSIONIDXXX).

Previously in the thread, I wanted to use the value found with
"capture cookie" but when i started to update the documentation, I
found this solution quite weird. I've made a small rework to not
depend on "capture cookie".

- There's the posssiblity to define the URL parser mode (path parameters
  or query string).