base: bug in copy constructor of Signal

The copy constructor of Signal did not copy the Signal::Data contents of
the copy source. This bug could survive undetected because the compiler
can optimize code in a way, that copy constructor and destructor are not
necessary when returning by value from simple functions. I assume that
it creates the object in CPU registers instead of RAM and reuses it
instead of copying it to save time. This way the bug triggered first
after wait_for_signal was changed in a way that avoided optimization.

ref #912