pgsql: Misc SCRAM code cleanups.

Misc SCRAM code cleanups.

* Move computation of SaltedPassword to a separate function from
scram_ClientOrServerKey(). This saves a lot of cycles in libpq, by
computing SaltedPassword only once per authentication. (Computing
SaltedPassword is expensive by design.)

* Split scram_ClientOrServerKey() into two functions. Improves
readability, by making the calling code less verbose.

* Rename "server proof" to "server signature", to better match the
nomenclature used in RFC 5802.

* Rename SCRAM_SALT_LEN to SCRAM_DEFAULT_SALT_LEN, to make it more clear
that the salt can be of any length, and the constant only specifies how
long a salt we use when we generate a new verifier. Also rename
SCRAM_ITERATIONS_DEFAULT to SCRAM_DEFAULT_ITERATIONS, for consistency.

These things caught my eye while working on other upcoming changes.

Branch
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master

Details
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https://git.postgresql.org/pg/commitdiff/d981074c24d2f1e4f44bc6d80e967e523ce64f50

Modified Files
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src/backend/libpq/auth-scram.c | 36 +++++++++++-----------
src/common/scram-common.c | 51 ++++++++++++++++++++-----------
src/include/common/scram-common.h | 15 ++++------
src/interfaces/libpq/fe-auth-scram.c | 58 +++++++++++++++++++-----------------
4 files changed, 89 insertions(+), 71 deletions(-)