Avoid resetting unique_ptr not being the owner of the pointed element

[whoan]

This PR is to avoid resetting a unique_ptr in a new thread as it (the new thread) is not the owner of the pointed element, and unique_ptr is not thread safe, so some obscure errors could happen. Consider this snippet for testing purposes:

#include <memory>
#include <thread>

int main() {
  std::unique_ptr<int> unique(new int);
  std::thread thread([&unique] () {
    unique.reset();
  });
  unique.reset();
  thread.join();
  return 0;
}

Both unique.reset(); might be called at the same time causing UB. I think something similar is happening in shutdown_wspp_impl.

What is the price of stop calling m_client.reset(); as suggested in this PR?

This might be related to issue #32

[BillyONeal]

Hmmm I don't think this is the case because it looks like the thread is joined before the call to reset, not after. This change doesn't seem to do anything other than consume more memory?

[whoan]

@BillyONeal I have seen some double deletions of the unique_ptr but I wonder if it was before PR #1080 was joined. I will try to reproduce later and will let you know.

[BillyONeal]

OK, feel free to reopen if you can run it down.