Address the important problem that previously, whenever the last Threads::Thread...

Address the important problem that previously, whenever the last Threads::Thread<> object went out of scope, the location where a thread would deposit its return value was in deleted memory and thus potentially very bad. We now handle this by making sure that the thread itself keeps a lock on the destruction of the object in which the return_value is located, by keeping another shared_ptr to it, and only releases the lock whenever the return value has been deposited.

git-svn-id: https://svn.dealii.org/trunk@18272 0785d39b-7218-0410-832d-ea1e28bc413d