the old implementation used threads for dispatching callouts, each callout
owned a thread and it suffered from a race where a callout structure could have
been deleted before the callout thread even begun.

the current implementation is dispatching all callouts in a single callout
dispatcher thread, it maintains an ordered list of callouts to achieve that.

this patch solve a crash with the TCPDownloadFile test, that now proceeds.

Most of this is just header shuffling.

It is already deduced using the masklen parameter

The EVENTHANDLER mechanism provides a way for kernel subsystems to register interest in kernel events and have their callback functions invoked when these events occur.