fixed: include boost header as necessary

Without this include, sometimes BOOST_VERSION was defined and sometimes
it was not, depending on which includes came before it. The result was a
random mix of sleep or sleep_for for boost versions >= 1.50.
fixed: don't use thread::sleep_for where it's known to be broken

Fixes #2690.
Merge pull request #2766 from theuni/fix-shutdown-deadlock

Fix shutdown deadlock, ticket #2690