Complex Query Support for Statistics

The rich query functionality, implemented by the Complex filter expressions in API queries blueprint, is available for Samples, Alarms and Alarm History, however this functionality is applicable for Statistics as well. The current implementation supports selectable parametrized aggregate functions (e.g. cardinality) with at most a single parameter per function. The idea is to provide the possibility of using the complex query functionality for Statistics and use the flexibility of the JSON representation of the request body for supporting the definition of multiple parameters for a selectable aggregate. This extension will also support all the functions that are available via the /meters endpoint with simple query.

As the rich query functionality uses new REST resources, Statistics will be implemented to fit in this new structure, with the following supported functions:
* /query/samples/statistics
** avg
** count
** max
** min
** sum
** cardinality
* /query/alarms/statistics
** count
* /query/alarms/history/statistics
** count

The implementation of the complex query provides the possibility to use orderby and limit for Samples, which will be supported in case of Statistics as well.

The use of POST request for complex queries makes it possible to define groupby with multiple values in one request. This feature is NOT equal to the SQL subqueries.