Projects/replay_cache_collision_avoidance and replay cache uses

[Nicolas Williams]

Back to hash agility for a moment.  As discussed, it's really hard to do
hash agility per-entry, and I won't propose that we do.

But it'd be nice if there was an rcache header entry naming the hash
function to use for that rcache.

New rcache implementations that support the hash named in the rcache
header entry should use it, otherwise they should act as old rcache
implementations.

Finally, allow for the hash function for new rcaches to be specified in
krb5.conf.

This gets us hash agility at little cost.  And not all of it need be
implemented now -- it suffices to have the header entry, with the rest
to be added later.

But NOT having such a header entry (naming a hash funciton, or rcache
version, or something) WILL complicate hash agility later.

Nico
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