DcpManagerSlave: join threads before delete in destructor

[oliverpapst1]

• ~DcpManagerSlave() deleted thread pointers without joining them first
• If a thread is still joinable at destruction time, std::thread::~thread() calls std::terminate(), causing the process to hang instead of exiting cleanly
• Added joinable() + join() before each delete across all managed threads

Closes issue #77.