Variable Granularity support for data collection (netdata#6430)

* Variable Granularity support for data collection in the dbengine.

* Variable Granularity support for data collection in the daemon.

* Added tests to validate the data being queried after having been collected by changing data collection interval

* Fix memory corruption

* Updated database engine documentation about data collection frequency behaviour

Author: mfundul

backends/backends.c

@@ -131,7 +131,8 @@ calculated_number backend_calculate_value_from_stored_data(
     }
 */
     for(rd->state->query_ops.init(rd, &handle, after, before) ; !rd->state->query_ops.is_finished(&handle) ; ) {
-        n = rd->state->query_ops.next_metric(&handle);
+        time_t curr_t;
+        n = rd->state->query_ops.next_metric(&handle, &curr_t);
 
         if(unlikely(!does_storage_number_exist(n))) {
             // not collected

daemon/unit_test.c

@@ -47,6 +47,8 @@ Currently Netdata supports 6 memory modes:
     database. There is some amount of RAM dedicated to data caching and indexing and the rest of
     the data reside compressed on disk. The number of history entries is not fixed in this case,
     but depends on the configured disk space and the effective compression ratio of the data stored.
+    This is the **only mode** that supports changing the data collection update frequency
+    (`update_every`) **without losing** the previously stored metrics.
     For more details see [here](engine/).
 
 You can select the memory mode by editing `netdata.conf` and setting:

database/README.md

@@ -4,6 +4,8 @@ The Database Engine works like a traditional
 database. There is some amount of RAM dedicated to data caching and indexing and the rest of
 the data reside compressed on disk. The number of history entries is not fixed in this case,
 but depends on the configured disk space and the effective compression ratio of the data stored.
+This is the **only mode** that supports changing the data collection update frequency
+(`update_every`) **without losing** the previously stored metrics.
 
 ## Files