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.
The only thing that is missing is a signature to detect the protocol (it
has no well-known port).
Reassembly is kind of fidgety - at the moment we only support
re-assembling one simultaneous message per direction (which looking at
our test-traffic might not be a problem). And I am not quite sure if I
got all cases correct...
But - it works :)
