MVStore: store the file header also at the end of each chunk, which means even less write operations

MVStore: unified exception handling; the version is included in the messages. Old data is now retained for 45 seconds by default.

Found by UCDetector, which seems to have gotten smarter recently.

And its fields are not accessed outside itself, so they can get marked as private.

Found by UCDetector, which seems to have gotten smarter recently.

fix: revert one one of my earlier cleanups - these fields are obviously meant to be public so that they are settable when we're doing development