moving basebackup code to its own directory

Hi,

I was thinking that it might make sense, to reduce clutter, to move
*backup*.c from src/backend/replication to a new directory, perhaps
src/backend/replication/backup or src/backend/backup.

There's no particular reason we *have* to do this, but there are 21 C
files in that directory and 11 of them are basebackup-related, so
maybe it's time, especially because I think we might end up adding
more basebackup-related stuff.

Thoughts?

-- 
Robert Haas
EDB: http://www.enterprisedb.com