…rk id (url@latest_block). if block provided then fork id is url@provided_block (this means there'll no longer be a url@latest fork id but always url@block)

- after creation of fork the id is checked if in inserted forks. If it doesn't exist then it's inserted, otherwise the existing backend is reused and a new fork url@latest_block-1 recorded

CreatedFork::inc_senders increments number of senders and returns the new unique fork id. CreatedFork::num_senders was removed
MultiForkHandler::insert_new_fork added to handle fork insertion / send on channels