SET default_tablespace not assign xid by avoiding to acquire lock.
SET default_tablespace not assign xid by avoiding to acquire lock.

SET default_tablespace ideally should just be read-only transaction and
shouldn't allocate xid. Currently get_tablespace_oid() is acquiring lock for
other purposes to protect for concurrency, ends up assigning transaction IDs to
take the lock. Setting the GUC doesn't need to acquire the lock, plus it creates
the problem as multiple readers and a writer in a gang all end-up assigning
independent xid when dispatched this GUC. This happens because SET runs as
QE_AUTO_COMMIT_IMPLICIT for writer and hence has deferred allocation of
transaction ID. The filespace ICW tests detected this case since hits assert
FailedAssertion("!(!((proc->xid) != ((TransactionId) 0)))", File: "procarray.c"
without the fix.

Simplest scenario the problem can be demostrated is:
- DROP TABLE a;
- CREATE TABLE a(a INT, b INT);
- INSERT INTO a VALUES (generate_series(1,2), generate_series(3,4)); -- generates 1 reader and 1 writer gang
- SET default_tablespace='pg_default';

cjcjameson/gpdb,Chibin/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,Quikling/gpdb,Quikling/gpdb,cjcjameson/gpdb,greenplum-db/gpdb,adam8157/gpdb,janebeckman/gpdb,edespino/gpdb,adam8157/gpdb,Quikling/gpdb,edespino/gpdb,jmcatamney/gpdb,50wu/gpdb,lisakowen/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,50wu/gpdb,yuanzhao/gpdb,janebeckman/gpdb,Chibin/gpdb,yuanzhao/gpdb,lisakowen/gpdb,rvs/gpdb,xinzweb/gpdb,greenplum-db/gpdb,jmcatamney/gpdb,greenplum-db/gpdb,lisakowen/gpdb,jmcatamney/gpdb,lisakowen/gpdb,rvs/gpdb,50wu/gpdb,xinzweb/gpdb,edespino/gpdb,kaknikhil/gpdb,Chibin/gpdb,lisakowen/gpdb,greenplum-db/gpdb,cjcjameson/gpdb,Quikling/gpdb,janebeckman/gpdb,janebeckman/gpdb,kaknikhil/gpdb,edespino/gpdb,edespino/gpdb,adam8157/gpdb,cjcjameson/gpdb,xinzweb/gpdb,ashwinstar/gpdb,edespino/gpdb,xinzweb/gpdb,Quikling/gpdb,50wu/gpdb,janebeckman/gpdb,janebeckman/gpdb,kaknikhil/gpdb,Quikling/gpdb,Quikling/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,adam8157/gpdb,ashwinstar/gpdb,50wu/gpdb,50wu/gpdb,greenplum-db/gpdb,kaknikhil/gpdb,yuanzhao/gpdb,jmcatamney/gpdb,rvs/gpdb,xinzweb/gpdb,edespino/gpdb,xinzweb/gpdb,cjcjameson/gpdb,greenplum-db/gpdb,yuanzhao/gpdb,janebeckman/gpdb,jmcatamney/gpdb,adam8157/gpdb,rvs/gpdb,yuanzhao/gpdb,greenplum-db/gpdb,xinzweb/gpdb,rvs/gpdb,cjcjameson/gpdb,jmcatamney/gpdb,Chibin/gpdb,edespino/gpdb,ashwinstar/gpdb,edespino/gpdb,janebeckman/gpdb,50wu/gpdb,jmcatamney/gpdb,rvs/gpdb,Quikling/gpdb,lisakowen/gpdb,cjcjameson/gpdb,rvs/gpdb,50wu/gpdb,yuanzhao/gpdb,janebeckman/gpdb,cjcjameson/gpdb,Chibin/gpdb,jmcatamney/gpdb,rvs/gpdb,edespino/gpdb,adam8157/gpdb,cjcjameson/gpdb,yuanzhao/gpdb,Chibin/gpdb,lisakowen/gpdb,Chibin/gpdb,kaknikhil/gpdb,Quikling/gpdb,xinzweb/gpdb,Chibin/gpdb,kaknikhil/gpdb,adam8157/gpdb,lisakowen/gpdb,rvs/gpdb,kaknikhil/gpdb,ashwinstar/gpdb,Quikling/gpdb,Chibin/gpdb,Chibin/gpdb,ashwinstar/gpdb,ashwinstar/gpdb,kaknikhil/gpdb,rvs/gpdb,janebeckman/gpdb,kaknikhil/gpdb,adam8157/gpdb