The Append / truncate operations were racy. When a datafile reaches 2Gb, a new file is needed. For this operation, we require a writelock, which is not needed in the 99.99% of all cases where the data does fit in the current head-file.

This transition from readlock to writelock was incorrect, and as the readlock was released, a truncate operation could slip in between, and truncate the data. This would have been fine, however, the Append operation continued writing as if no truncation had occurred, e.g writing item 5 where item 0 should reside.

This PR changes the behaviour, so that if when we run into the situation that a new file is needed, it aborts, and retries, this time with a writelock.

The outcome of the situation described above, running on this PR, would instead be that the Append operation exits with a failure.