journal: use a different hash function for each journal file

journal: use a different hash function for each journal file

This adds a new (incompatible) feature to journal files: if enabled the
hash function used for the hash tables is no longer jenkins hash with a
zero key, but siphash keyed by the file uuid that is included in the
file header anyway. This should make our hash tables more robust against
collision attacks, as long as the attacker has no read access to the
journal files. We switch from jenkins to siphash simply because it's
more well-known and we standardize for the rest of our codebase onto it.

This is hardening in order to make collision attacks harder for clients
that can forge log messages but have no read access to the logs. It has
no effect on clients that have read access.