Add experimental support for 5sec granularity for host timeseries

To enable it (only on *InfluxDB* right now):

- uncomment HOST_NUM_TIMESERIES_POINTS in ntop_defines.h
- run `redis-cli set ntopng.prefs.30_sec_dump 1` and restart ntopng

scripts/lua/modules/timeseries/drivers/sample.lua

@@ -57,9 +57,10 @@ function driver:export()
 end
 
 --! @brief Get the most recent timestamp available for queries.
+--! @param ifid can be used to possibly provide a more accurate answer.
 --! @note a conservative way to implement this is to return the current time.
 --! @return most recent timestamp available.
-function driver:getLatestTimestamp()
+function driver:getLatestTimestamp(ifid)
 end
 
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