Align global space dbOid with the backend (#275)

PG backend uses `InvalidOid` as the database Oid for global space rel
locators (see: https://git.postgresql.org/gitweb/?p=postgresql.git;a=blob;f=src/backend/utils/cache/relcache.c;h=930cc03ee20b10262815f7c879494e2576593882;hb=b18b3a8150dbb150124bd345e000d6dc92f3d6dd#l1333).
So should we.

Otherwise, it can (and does) cause issues like
https://perconadev.atlassian.net/browse/PG-1000
In that case, the backend creates a temporary relation in the "global space"
leading to a dbOid mismatch.

Fixes PG-1000

src/include/catalog/tde_global_space.h

@@ -21,9 +21,10 @@
  * We take Oids of the sql operators, so there is no overlap with the "real"
  * catalog objects possible.
  */
-#define GLOBAL_DATA_TDE_OID	607 /* Global objects fake "db" */
 #define XLOG_TDE_OID        608
 
+#define GLOBAL_DATA_TDE_OID	InvalidOid
+
 #define GLOBAL_SPACE_RLOCATOR(_obj_oid) (RelFileLocator) { \
 	GLOBALTABLESPACE_OID, \
 	GLOBAL_DATA_TDE_OID, \