Improved cookie and headers handling

* It was possible to access cookies from previous and parallel requests; now cookiejars are unique.
* `headers` argument of render.* endpoint was setting headers for all outgoing requests, not only for the first request. This is changed to match documented behaviour.
* New methods to handle cookies from Lua scripts: 
  * splash:add_cookie()
  * splash:get_cookies()
  * splash:init_cookies()
  * splash:clear_cookies()
  * splash:delete_cookies()
* splash:set_custom_headers() method for setting default headers for all requests 
* `headers` argument for splash:go() which allows to set headers for the initial request
* improved arguments validation for splash:go()
* splash:set_user_agent() method