Add --if-not-exists flag to receive-wal

[crazybolillo]

This flag prevents receive-wal from exiting with an error when a slot with the same name already exists. This is the same behavior as calling pg_receivewal with the same flag.

References: #1049.

[crazybolillo]

I'm antonag32 but ended up doing this outside work hours so I'm using my personal account. Let me know if I have to make any changes.

[crazybolillo]

Added the new flag to the RST docs.

[martinmarques]

Hi, we pulled this branch into our internal infrastructure for testing. It's ready for release which should be pretty soon. Closing this PR now