statistics: internal statistics' data moved out from statistics class declaration (pimpl idiom).
serialization: statistics data serialization to rapidjson added.
backends: file_logger added.
benchmarks/load: file_logger support added.
benchmarks/load: run_load.sh script calls load benchmark with file_logger options.
serialization: statistics deserialization from rapidjson added.
statistics: truncating mechanics for statistics data added.
statistics: compress_histogram function added.
statistics: append(data) method added.
statistics: data's fulfill_dependencies() method added.
statistics: constructor from statistics data added.
statistics: enabled() check must also check for computed().

computed() checks for statistics data tags and shows if particular statistics tag can be retrieved.
enabled() checks if particular tag was requested by user (but it also should be computed).
utils: log-aggregate util added.