mgr: only include necessary headers

mgr: only include necessary headers

for two reasons:

* faster compilation with less source files to process
* avoid including "boost/detail/iterator.hpp", so we can
  silence the warning like:

boost/detail/iterator.hpp:13:1: note: ‘#pragma message: This header is
deprecated. Use <iterator> instead.’
   13 | BOOST_HEADER_DEPRECATED("<iterator>")
      | ^~~~~~~~~~~~~~~~~~~~~~~

Signed-off-by: Kefu Chai <kchai@redhat.com>