Enhancement: elasticsearch time-based indices

[faxm0dem]

Add a timestamping option that accepts either {daily,weekly,monthly,yearly) or a indexsuffix that would accept stftime type formatting e.g. -%y.%m.%d so we can have UTC dependant indices.
This cannot at the time be handled in syslog-ng because $YEAR, $MONTH, and $DAY macros are not UTC but local TZ

[faxm0dem]

For the record, this is the logic using the perl plugin: https://github.com/faxm0dem/syslog_ng-elasticsearch/blob/master/perl/Elasticsearch.pm#L66-L71

[faxm0dem]

as pointed out by @fekete-robert if global option time-zone(UTC) is set, being able to pass index("myindex-$YEAR.$MONTH.$DAY") would do the job, but @juhaszviktor and @talien said implementing templates for the queue function is difficult