Optimize local time functions for the UTC case

Currently two flags decide whether to do UTZ->local timezone conversions

• convert_legacy_hive_parquet_utc_timestamps
• use_local_tz_for_unix_timestamp_conversions
  Enabling these global flags make Impala more compatible with Hive, but can highly degrade performance. Since IMPALA-7362, it is possible to override the local timezone with a query option. Setting timezone to UTC practically disables the flags above, but their overhead does not disappear completely. It would be relatively easy to create shortcuts for the UTC case to avoid this overhead.

This could make it easier to choose the right values for the flags: both could be set to true, and users who want to avoid the performance impact could switch to UTC.