The ThreadSpanLinks singleton holds the active span (if one exists) for
a given thread ID. The `get_active_span_from_thread_id` member function
returns a pointer to the active span for a thread. The `link_span`
member function sets the active span for a thread.
`get_active_span_from_thread_id` accesses the map of spans under a
mutex, but returns the pointer after releasing the mutex, meaning
`link_span` can modify the members of the Span while the caller of
`get_active_span_from_thread_id` is reading them.

Fix this by returning a copy of the `Span`. Use a `std::optional` to wrap
the return value of `get_active_span_from_thread_id`, rather than
returning a pointer. We want to tell whether or not there actually was a
span associated with the thread, but returning a pointer would require
us to heap allocate the copy of the Span.

I added a simplistic regression test which fails reliably without this
fix when built with the thread sanitizer enabled. Output like:

```
WARNING: ThreadSanitizer: data race (pid=2971510)
  Read of size 8 at 0x7b2000004080 by thread T2:
    #0 memcpy ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:823 (libtsan.so.0+0x42313)
    #1 memcpy ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:815 (libtsan.so.0+0x42313)
    #2 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > const&) <null> (libstdc++.so.6+0x1432b4)
    #3 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::__invoke_impl<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()>(std::__invoke_other, std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*&&)()) <null> (thread_span_links+0xe46e)
    #4 std::__invoke_result<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()>::type std::__invoke<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()>(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*&&)()) <null> (thread_span_links+0xe2fe)
    #5 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > std::thread::_Invoker<std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) <null> (thread_span_links+0xe1cf)
    #6 std::thread::_Invoker<std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()> >::operator()() <null> (thread_span_links+0xe0f6)
    #7 std::thread::_State_impl<std::thread::_Invoker<std::tuple<std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> > (*)()> > >::_M_run() <null> (thread_span_links+0xdf40)
    #8 <null> <null> (libstdc++.so.6+0xd6df3)

  Previous write of size 8 at 0x7b2000004080 by thread T1 (mutexes: write M47):
    #0 memcpy ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:823 (libtsan.so.0+0x42313)
    #1 memcpy ../../../../src/libsanitizer/sanitizer_common/sanitizer_common_interceptors.inc:815 (libtsan.so.0+0x42313)
    #2 std::__cxx11::basic_string<char, std::char_traits<char>, std::allocator<char> >::_M_assign(std::__cxx11::basic_string<char, std::char_traits<char>, std::allocato
r<char> > const&) <null> (libstdc++.so.6+0x1432b4)
    #3 get() <null> (thread_span_links+0xb570)
    #4 void std::__invoke_impl<void, void (*)()>(std::__invoke_other, void (*&&)()) <null> (thread_span_links+0xe525)
    #5 std::__invoke_result<void (*)()>::type std::__invoke<void (*)()>(void (*&&)()) <null> (thread_span_links+0xe3b5)
    #6 void std::thread::_Invoker<std::tuple<void (*)()> >::_M_invoke<0ul>(std::_Index_tuple<0ul>) <null> (thread_span_links+0xe242)
    #7 std::thread::_Invoker<std::tuple<void (*)()> >::operator()() <null> (thread_span_links+0xe158)
[ ... etc ... ]
```