Allow using replication origins in SQL level parallel sessions

Replication origins is a useful feature for external replication
systems to do conflict resolution in bi-directional replication.  It's
possible for the external systems to make use of this feature using
the SQL functions pg_replication_origin_*().

pg_replication_origin_session_setup() is the one to configure the
current session.  Currently, only a single version of this function is
exposed that allows one replica origin to be used only by a single
session.  This limits the usefulness of this feature.

The attached patch creates another variant of this function
pg_replication_origin_session_setup(text, int).  This allows the same
replica origin to be used by parallel sessions the same way logical
replication apply workers are using it.

I'll add this to the next commitfest.