Capture required catalogs information

[rjuju]

Naive initial list:

• pg_class(oid, relname, relnamespace, relkind)
• pg_attribute(oid, attname, attrelid, atttypid)
• pg_namespace(oid, nspname)
• pg_type(oid, typname)

This obviously shouldn't been for all databases and all snapshots, so some heuristics will have to be added to know when to perform that. Some ideas for that:

• Cache the current ShmemVariableCache->nextOid for each server and the highest recorded oid found in each database
• let user ask for refreshing that data from the UI, propagated to the collector
• use ProcessUtility_hook to detect when handled objects have been created or dropped

[rjuju]

Per-database catalog snapshot and a button to force a refresh from the UI have been added in the v5 version, so closing this issue.