Feat[MQB]: Json printer can print clients and clusters stat context data

[bbpetukhov]

Describe your changes

• Stat printer can output stat contexts snapshot as ascii table (as before) and also in JSON format to stat log file. TABLE/JSON/TABLE_AND_JSON options can be selected in broker config file.
• JSON stat printer outputs stats also for clients, channels and allocators stat contexts. JSON structure changed to be list of flat objects.
• Response of JSON admin command STAT SHOW changed accordingly.

Testing performed

• Updated admin command (STAT SHOW) integration tests