On-disk format of SCRAM verifiers

The current format for SCRAM verifiers in pg_authid is:

scram-sha-256:<salt>:<iteration count>:<StoredKey>:<ServerKey>

While reviewing Michael's patch to change that so that StoredKey and
ServerKey are stored base64-encoded, rather than hex-encoded as they are
currently [1], I looked again at RFC 5803. RFC 5803 specifies the format
to use when storing SCRAM verifiers in LDAP. I looked at it earlier, and
it was a source of inspiration for the current format, but I didn't
think that it was directly applicable. I thought that in RFC 5803 the
different fields were stored as separate fields or attributes, not as a
single string.

But looking more closely, I think I misunderstood RFC 5803. It *does* in
fact specify a single string format to store the verifier in. And the
format looks like:

SCRAM-SHA-256$<iteration count>:<salt>$<StoredKey>:<ServerKey>

Alternating '$' and ':' as the separators seems a bit wonky, but it
actually makes sense. "<iteration count>:<salt>" is treated as one
field, and "<StoredKey>:<ServerKey>" is treated as another, which is
logical, since the iteration count and salt are sent together to the
client in the SCRAM challenge, while StoredKey and ServerKey must be
kept secret.

I think we should adopt that exact format, so that our verifiers are
compatible with RFC 5803. It doesn't make any immediate difference, but
since there is a standard out there, might as well follow it. And just
in case we get support for looking up SCRAM verifiers from an LDAP
server in the future, it will come handy as we won't need to parse two
different formats.

Barring objections, I'll go change our on-disk format for SCRAM
verifiers to follow RFC 5803.

[1]
https://www.postgresql.org/message-id/CAB7nPqSbsCBCxy8-DtwzRxYgTnbGUtY4uFEkLQhG%3DR%3Duo%3Dg8Fw%40mail.gmail.com

- Heikki