Prevent timestamps in node properties from being shifted

according to the timezone WriteNodeProperties is being run in.

Due to our ORC exports using the `timestamp` instead of the `timestamp with timezone`, reader and writer need to agree out of bound on the timezone used in files they exchange.

However, we don't do this:

* `swh-dataset` uses pyorc, which uses the C++ ORC library, which assumes users (us) always write in GMT
* `swh-graph` uses the Java ORC library, which assumes the system timezone (or `$TZ` if set)

So when reading with a non-UTC timezone, the Java ORC library interprets timestamps in the dataset as being in the local timezone, and converts them to UNIX timestamps (number of seconds since epoch); then we use these converted timestamps and write them to `.property.author_timestamp.bin` and `.property.committer_timestamp.bin`.

This commit regenerates the example graph to have the correct timestamps. It also applies the 39ed0d17 change that removes useless padding at the end of all property files.

Resolves #4788