Allow sql_last_value to be used without timezone

[jsvd]

By default, when using sql_last_value with a timestamp, the TIMESTAMP WITH TIMEZONE type is used in oracle databases, so that the timezone is always included.
Some users want to use a TIMESTAMP value instead, not wanting the timezone to be compared.

Currently both time trackers in https://github.com/logstash-plugins/logstash-integration-jdbc/blob/master/lib/logstash/plugin_mixins/jdbc/value_tracking.rb#L74-L100 generate times with timezones, so a new configuration is needed to toggle the use of timezones