fix: use nanoseconds in Header.Time everywhere

[keruch]

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[danwt]

Just did a quick scan
will take your word for it

[keruch]

regarding backwards-compatibility of changing time from uint64 to int64
https://protobuf.dev/programming-guides/proto3/#updating

int32, uint32, int64, uint64, and bool are all compatible – this means you can change a field from one of these types to another without breaking forwards- or backwards-compatibility. If a number is parsed from the wire which doesn’t fit in the corresponding type, you will get the same effect as if you had cast the number to that type in C++ (for example, if a 64-bit number is read as an int32, it will be truncated to 32 bits).