PATCH: AM-specific statistics, with an example implementation for BRIN (WIP)

failed: regress/opr_sanity, regress/vacuum, regress/brin, regress/brin_bloom, regress/brin_multi, regress/sysviews, regress/stats_ext, regress/select_parallel, regress/write_parallel, regress/vacuum_parallel, regress/publication, regress/subscription, regress/select_views, regress/portals_p2, regress/foreign_key, regress/cluster, regress/dependency, regress/guc, regress/bitmapops, regress/combocid, regress/tsearch, regress/tsdicts, regress/foreign_data, regress/window, regress/xmlmap, regress/functional_deps, regress/advisory_lock, regress/indirect_toast, regress/equivclass, regress/json, regress/jsonb, regress/json_encoding, regress/jsonpath, regress/jsonpath_encoding, regress/jsonb_jsonpath, regress/plancache, regress/limit, regress/plpgsql, regress/copy2, regress/temp, regress/domain, regress/rangefuncs, regress/prepare, regress/conversion, regress/truncate, regress/alter_table, regress/sequence, regress/polymorphism, regress/rowtypes, regress/returning, regress/largeobject, regress/with, regress/xml, regress/partition_join, regress/partition_prune, regress/reloptions, regress/hash_part, regress/indexing, regress/partition_aggregate, regress/partition_info, regress/tuplesort, regress/explain, regress/compression, regress/memoize, regress/stats, regress/event_trigger, regress/oidjoins, regress/fast_default, tap/002_pg_upgrade, pg_upgrade/002_pg_upgrade