Implement Preference to Dump pcap-Flows on Clickhouse

[lucaderi]

As of today, when ntopng reads a pcap file, the generated flows are not written to ClickHouse. This is to request the implementation of a new preference (disabled by default) that will also write to ClickHouse the flows produced by ntopng.

Implemented the preference, the ntopng engine should be updated

[MatteoBiscosi]

The preference seems okay but the historical is not accessible from a pcap interface and if i try to force it an error pops up

[cardigliano]

This seems to be failing on db_search_manager.get_earliest_available_epoch()

[lucaderi]

Verified as fixed