log message "error loading snapshot" reported under simple workload

[mschoch]

@rubiojr reports here #39 that when using a Reader not obtained from the Writer and while the Writer is still open, the message "error loading snapshot epoch: %d: %v" appears.

[mschoch]

One theory is that as the Writer independently creates new snapshots, when opening a Reader (not via the Writer) it may attempt to open a partially written snapshot file. This would fail the CRC check, and another snapshot is tried.

We should be able to test this theory with a straightforward unit test.

One approach to fix this might be to write new snapshot files with a different extension, and rename them after a complete snapshot has been written. This would prevent new readers from seeing partially written snapshot files.