replace boost::thread with std::thread

[Peter]

It seems thread is not exposed in the public (or protected) API, which would allow us to replace all boost::thread with std::thread. If not for other reasons, it will avoid complexity linking to 3rd party libraries.

[Peter]

In 3946:

use mutex and locks from std:: rather than boost::. refs #953

[Peter]

In 3948:

replace boost::thread with std::thread in Scheduler. refs #953

[Peter]

In 3949:

use std::thread rather than boost::thread in test, refs #953

[Peter]

In 3959:

use std::mutex and friends instead of boost variants. refs #953

[Peter]

In 3960:

closes #953. Since we are no longer using libboost_thread (or
underlying libbost_system), stop linking agaist them. Instead look for
needed compiler and extra LIBS needed for thread support.