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.cpp

@@ -311,11 +311,8 @@ void ProjectFileIO::CheckpointThread()
          }
 
          // Capture the number of pages that need checkpointing and reset
-         mCheckpointCurrentPages = mCheckpointWaitingPages;
+         mCheckpointCurrentPages.store( mCheckpointWaitingPages );
          mCheckpointWaitingPages = 0;
-
-         // Lock out others while the checkpoint is running
-         mCheckpointActive.lock();
       }
 
       // Open another connection to the DB to prevent blocking the main thread.
@@ -334,10 +331,10 @@ void ProjectFileIO::CheckpointThread()
 
          // Reset
          mCheckpointCurrentPages = 0;
-
-         // Checkpoint is complete
-         mCheckpointActive.unlock();
       }
+      else
+         // Gotta close it anyway!
+         sqlite3_close( db );
    }
 
    return;
@@ -348,7 +345,7 @@ int ProjectFileIO::CheckpointHook(void *data, sqlite3 *db, const char *schema, i
    // Get access to our object
    ProjectFileIO *that = static_cast<ProjectFileIO *>(data);
 
-   // Qeuue the database pointer for our checkpoint thread to process
+   // Queue the database pointer for our checkpoint thread to process
    std::lock_guard<std::mutex> guard(that->mCheckpointMutex);
    that->mCheckpointWaitingPages = pages;
    that->mCheckpointCondition.notify_one();