Set default timezone to UTC in importer

[morazow]

Situation

Currently, when importing timestamp values using java.sql.Timestamp, they are converted to the timezone of the database nodes (DBTIMEZONE setting). This leads to the wrong expectations, since Timestamp is always UTC.

Until this this feature is introduced exasol/script-languages-release#539, set the UDF application timezone to UTC using:

TimeZone.setDefault(TimeZone.getTimeZone("UTC"))

Acceptance Criteria

• UTC timezone is set before emitting
• Test adapted

[morazow]

Check that EXPORT is also fixed and behaves similarly.

[morazow]

As discussed with @allipatev, we should enable this with user parameter first so that users are not faced with sudden behavior change.