Unitary thread review (#623)

* Fix uninitialized members of ProjectFileIO...

... which fixes this problem I observed:

Opening a previously saved project, saving-as to another path, then exiting
Audacity, gives a progress dialog waiting for checkpoints to end, which doesn't
go away

* Remove two mutexes...

... One wasn't used at all, and another was only ever used by one thread, and
then not correctly unlocked for each locking on all possible paths.

* Values that the worker thread writes and main reads should be atomic

* Remember to close db connections even after failure to open

src/ProjectFileIO.h

@@ -229,11 +229,9 @@ private:
    std::thread mCheckpointThread;
    std::condition_variable mCheckpointCondition;
    std::mutex mCheckpointMutex;
-   std::mutex mCheckpointActive;
-   std::mutex mCheckpointClose;
-   std::atomic_bool mCheckpointStop;
-   uint64_t mCheckpointWaitingPages;
-   uint64_t mCheckpointCurrentPages;
+   std::atomic_bool mCheckpointStop{ false };
+   std::atomic< std::uint64_t > mCheckpointWaitingPages{ 0 };
+   std::atomic< std::uint64_t > mCheckpointCurrentPages{ 0 };
 
    friend SqliteSampleBlock;
    friend AutoCommitTransaction;