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

src/backend/bootstrap/bootparse.y

@@ -209,6 +209,9 @@ Boot_CreateStmt:
 
 					if ($4)
 					{
+						TransactionId relfrozenxid;
+						MultiXactId relminmxid;
+
 						if (boot_reldesc)
 						{
 							elog(DEBUG4, "create bootstrap: warning, open relation exists, closing first");
@@ -226,7 +229,9 @@ Boot_CreateStmt:
 												   RELPERSISTENCE_PERMANENT,
 												   shared_relation,
 												   mapped_relation,
-												   true);
+												   true,
+												   &relfrozenxid,
+												   &relminmxid);
 						elog(DEBUG4, "bootstrap relation created");
 					}
 					else