Add Comments (for Bug 2673 fix)

Bug 2673 was an important multi-threading issue, and so I added
comments about the code.  Also a LogDebug to track cache use.

Bug 2673 was also a crash with no error message reported.
That is something to revisit later and deserved a comment.
It's believed that the underlying issue is fixed now.

The 'ANSWER-MEs' can be answered in 3.0.1.

src/SqliteSampleBlock.cpp

@@ -571,6 +571,13 @@ size_t SqliteSampleBlock::GetBlob(void *dest,
 
       // Just showing the user a simple message, not the library error too
       // which isn't internationalized
+      // Actually this can lead to 'Could not read from file' error message
+      // but it can also lead to no error message at all and a flat line, 
+      // depending on where GetBlob is called from.
+      // The latter can happen when repainting the screen.
+      // That possibly happens on a very slow machine.  Possibly that's the
+      // right trade off when a machine can't keep up?
+      // ANSWER-ME: Do we always report an error when we should here?
       Conn()->ThrowException( false );
    }