Request: new methods to validate socket request and request context.

this patch adds two new methods on Request object to validate the
requests context by socket number of dr:

   int validate_socket(int socket);
   int validate_request(duda_request_t *dr);

if there is an active connection associated they return MK_TRUE,
otherwise MK_FALSE.

Signed-off-by: Eduardo Silva <edsiper@gmail.com>