Split QAN data collection protocol in two halves: pmm-agent<->pmm-managed and pmm-managed<->pmm-agent. Do not change the later part at this ticket. For the former part:
- extract DBMS-specific fields into own sub-messages;
- maybe extract common field into own sub-message;
- add a list of table names as a common field; set it for PostgreSQL;
- use new protocol in pmm-agent;
- convert buckets in pmm-managed.