On shutdown some threads would continue to run after or during a pointer reset. This leads to occasional segfaults on shutdown.

Fix this by resetting the smart pointers after all threads that might read from them have been stopped.

This should fix:
* A segfault in the txindex thread, that occurs when the txindex destructor is done, but the thread was not yet stopped (as this is done in the base index destructor)
* A segfault in the scheduler thread, which dereferences conman. (e.g. CheckForStaleTipAndEvictPeers)