pgsql: tableam: relation creation, VACUUM FULL/CLUSTER,SET TABLESPACE.

tableam: relation creation, VACUUM FULL/CLUSTER, SET TABLESPACE.

This moves the responsibility for:
- creating the storage necessary for a relation, including creating a
  new relfilenode for a relation with existing storage
- non-transactional truncation of a relation
- VACUUM FULL / CLUSTER's rewrite of a table
below tableam.

This is fairly straight forward, with a bit of complexity smattered in
to move the computation of xid / multixid horizons below the AM, as
they don't make sense for every table AM.

Author: Andres Freund
Discussion: https://postgr.es/m/20180703070645.wchpu5muyto5n647@alap3.anarazel.de

Branch
------
master

Details
-------
https://git.postgresql.org/pg/commitdiff/d25f519107bff602e1ebc81853fe592d020c118d

Modified Files
--------------
src/backend/access/heap/heapam_handler.c | 451 +++++++++++++++++++++++++++++++
src/backend/bootstrap/bootparse.y        |   7 +-
src/backend/catalog/heap.c               | 120 ++++----
src/backend/catalog/index.c              |  11 +-
src/backend/catalog/storage.c            |  88 ++++++
src/backend/commands/cluster.c           | 342 ++---------------------
src/backend/commands/sequence.c          |  30 +-
src/backend/commands/tablecmds.c         | 180 ++++--------
src/backend/utils/cache/relcache.c       |  77 +++---
src/include/access/tableam.h             | 117 ++++++++
src/include/catalog/heap.h               |   6 +-
src/include/catalog/storage.h            |   3 +
src/include/utils/relcache.h             |   3 +-
13 files changed, 856 insertions(+), 579 deletions(-)