MEDIUM: fd: rely more on fd_update_events() to detect changes

This function already performs a number of checks prior to calling the
IOCB, and detects the change of thread (FD migration). Half of the
controls are still in each poller, and these pollers also maintain
activity counters for various cases.

Note that the unreliable test on thread_mask was removed so that only
the one performed by fd_set_running() is now used, since this one is
reliable.

Let's centralize all that fd-specific logic into the function and make
it return a status among:

  FD_UPDT_DONE,        // update done, nothing else to be done
  FD_UPDT_DEAD,        // FD was already dead, ignore it
  FD_UPDT_CLOSED,      // FD was closed
  FD_UPDT_MIGRATED,    // FD was migrated, ignore it now

Some pollers already used to call it last and have nothing to do after
it, regardless of the result. epoll has to delete the FD in case a
migration is detected. Overall this removes more code than it adds.
8 files changed