Sanitize the prototype of Actor::on_exit() callbacks

Sanitize the prototype of Actor::on_exit() callbacks

Changes on_exit(function<void(int failed, void*data)>, data)
into    on_exit(function<void(bool failed)>)

So, we are using bool for boolean values, and we don't force the user
to build a closure on void* data manually. If you want to pass a data
to the callback, just use the C++ formalism: [data](bool failed){...}

Other changes on the way (that excalated quickly)
- Document some recent changes to the internals in ChangeLog
- Document this_actor::on_exit() and Actor::on_exit() in doxygen
- Also sanitize the prototype of MSG_process_on_exit(). Was taking a
  (void*,void*) function; now takes a (int, void*) one. This was too
  weird to use void* to denote integers -- I actually doubt it ever
  worked the way it was written, but the tests are too poor to be
  sure: the only usage of this function ignores the return value.
- Also add a new test in activity-lifetime: test_sleep_restart_end
  Commented for now as it breaks miserably.