Replace MD5 use in rcache with SHA-256

Replace MD5 use in rcache with SHA-256

The rcache implementation uses an unkeyed MD5 hash of the
authenticator to distinguish between different requests with equal
client principal, server principal, and microsecond time.  When the
OpenSSL crypto provider is used and the underlying OpenSSL library is
run in FIPS mode, the MD5 algorithm is disabled and
gss_accept_sec_context() results in an abort in rcache processing.

This change effectively implements a different rcache extension.
The new extension identifier is 'SHA256:' (instead of 'HASH:')
and the new has algorithm is SHA-256.

ticket: 8353 (new)