pg_receivewal starting position

Hello,

I've notived that pg_receivewal logic for deciding which LSN to start
streaming at consists of:
- looking up the latest WAL file in our destination folder, and resume from
here
- if there isn't, use the current flush location instead.

This behaviour surprised me when using it with a replication slot: I was
expecting it to start streaming at the last flushed location from the
replication slot instead. If you consider a backup tool which will take
pg_receivewal's output and transfer it somewhere else, using the replication
slot position would be the easiest way to ensure we don't miss WAL files.

Does that make sense ?

I don't know if it should be the default, toggled by a command line flag, or if
we even should let the user provide a LSN.

I'd be happy to implement any of that if we agree.

--
Ronan Dunklau