Attached patch implements wal_sync_method as an enum. I'd like someone
to look it over before I apply it - I don't have the machines to test
all codepaths (and some of it is hard to test - better to read it and
make sure it's right).
In order to implement the enum guc, I had to break out a new
SYNC_METHOD_OPEN_DSYNC out of SYNC_METHOD_OPEN where it was previously
overloaded. This is one of the parts where I'm slightly worried I have
made a mistake.
//Magnus
