Arrange to copy relcache's trigdesc structure at the start of any

Arrange to copy relcache's trigdesc structure at the start of any
query that uses it.  This ensures that triggers will be applied consistently
throughout a query even if someone commits changes to the relation's
pg_class.reltriggers field meanwhile.  Per crash report from Laurette Cisneros.
While at it, simplify memory management in relcache.c, which no longer
needs the old hack to try to keep trigger info in the same place over
a relcache entry rebuild.  (Should try to fix rd_att and rewrite-rule
access similarly, someday.)  And make RelationBuildTriggers simpler and
more robust by making it build the trigdesc in working memory and then
CopyTriggerDesc() into cache memory.