This commit switches UID hashing from md5 to a highway hash. It also
moves the salt value out of the file plugin - and makes it
installation-specific instead - it is moved to the global namespace.
There now are digest hash functions to make "static"
installation-specific hashes that are stable over workers available to
everyone; hashes can be 64, 128 or 256 bits in size.
Due to the fact that we switch the file hashing algorithm, all file
hashes change.
The underlyigng algorithm that is used for hashing is highwayhash-128,
which is significantly faster than md5.
There was a bug in the new parsing code, introduced in
708ede22c6 which parses validity times
incorrectly if they are before the year 2000. What happens in this case
is that the 2-digit year will be interpreted to be in the 21st century
(1999 will be parsed as 2099, e.g.).
