Fixed possible memory order bug

[Evgeny-S-Sorokin]

Describe the changes

I believe, there was a memory order bug, concerning usage of atomic flags before condition_variable.notify.
According to https://en.cppreference.com/w/cpp/thread/condition_variable

Even if the shared variable is atomic, it must be modified under the mutex in order to correctly publish the modification to the waiting thread.

So I fenced both flags and notify statement with mutex.

Testing

Have you tested the new code using the provided automated test program and/or performed any other tests to ensure that it works correctly? If so, please provide information about the test system(s):

Yes, I did.

• CPU model, architecture, # of cores and threads: Intel(R) Core(TM) i5-7200U CPU @ 2.50GHz 2.71 GHz, 2 cores, 4 threads
• Operating system: Windows 10 Pro, x64
• Name and version of C++ compiler: Visual Studio 19 compiler
• Full command used for compiling, including all compiler flags: I don't think it matters.

Additional information

When I used Your project as a template for my personal project, I had a bug where threads would be notified before end flag set to true causing deadlock. It was captured using high-end system and I could not replicate it on my personal computer.

[bshoshany]

Thanks for the PR, @Evgeny-S-Sorokin!. This seems like a necessary modification, based on the quote from cppreference. However, I prefer to use the C++17 std::scoped_lock instead of std::lock_guard. I will incorporate this into the next release (which should be published in a few weeks).

[bshoshany]

Update: This issue has been fixed in v3.5.0, which was just released. (I did it in a slightly different way than what you suggested.) Thanks again!