Generalize concept of temporary relations to "relation persistence".

This commit replaces pg_class.relistemp with pg_class.relpersistence;
and also modifies the RangeVar node type to carry relpersistence rather
than istemp.  It also removes removes rd_istemp from RelationData and
instead performs the correct computation based on relpersistence.

For clarity, we add three new macros: RelationNeedsWAL(),
RelationUsesLocalBuffers(), and RelationUsesTempNamespace(), so that we
can clarify the purpose of each check that previous depended on
rd_istemp.

This is intended as infrastructure for the upcoming unlogged tables
patch, as well as for future possible work on global temporary tables.

src/backend/bootstrap/bootparse.y

@@ -219,6 +219,7 @@ Boot_CreateStmt:
 												   $3,
 												   tupdesc,
 												   RELKIND_RELATION,
+												   RELPERSISTENCE_PERMANENT,
 												   shared_relation,
 												   mapped_relation,
 												   true);
@@ -238,6 +239,7 @@ Boot_CreateStmt:
 													  tupdesc,
 													  NIL,
 													  RELKIND_RELATION,
+													  RELPERSISTENCE_PERMANENT,
 													  shared_relation,
 													  mapped_relation,
 													  true,