CREATE_REPLICATION_SLOT exported snapshot

Hi,

There's a brief mention of CREATE_REPLICATION_SLOT exporting a snapshot
here:


https://www.postgresql.org/docs/devel/static/logicaldecoding-explanation.html#AEN71570

but how to access the snapshot identifier is not documented anywhere.
So I came up with the attached patch (against master).  Please have a
looksie.


.m