Specify that timestamps are uint64

Copied from @ConradIrwin 's comments on #1 

> Currently the time of a change is specified to be an int64 in a change chunk, but a uint64 in a document chunk. (Also the 0 value, should really be interpreted as null). It won't matter for a few hundred million years (or until time-travel to pre-1970 is legalized), but it's probably easy to redefine the time to be uint64 everywhere.