MINOR: connection: use uint64_t for the hashes

MINOR: connection: use uint64_t for the hashes

The hash type stored everywhere is XXH64_hash_t, which annoyingly forces
everyone to include the huge xxhash file. We know it's an uint64_t because
that's its purpose and the type is only made to abstract it on machines
where uint64_t is not availble. Let's switch the type to uint64_t
everywhere and avoid including xxhash from the type file.