Issue 3797: Music_wrapper_iterator does not follow original context setting of child

Issue 3797: Music_wrapper_iterator does not follow original context setting of child

We try doing this with a descend_to_child (child_iter->get_outlet ())
after initializing the child iterator.  It's not clear that this will
work in all cases, but at least it addresses the reported issue where:

\score {
  <<
    { { a4 a b b } a a b b }
    { \absolute { a4 a b b } a a b b }
  >>
}

splits off a new Staff after \absolute { a4 a b b }.

[dak, 2014-01-11 19:11:19]

Use virtual set_context/get_handle to refrain from duplicate contexts

[dak, 2014-01-11 22:35:50]

Make a few safety related fixes, in particular to avoid early gc