Add a protobuf to represent time with integer precision.

Existing timestamps in the protobufs use double to encode time. Generally, the field represents seconds (with the decimal component to represent smaller denominations of time). This is less than ideal.

Instead, we should use integers, so as to not lose data (and to be able to compare value reliably).

Something like:

message Time {
  int64 seconds;
  int32 nanoseconds;
}