Revisit timestamp and duration representation

The current proto representation of timesetamp and durations in Beam is either raw int64s or the well-known Google protobuf types "google.protobuf.timestamp" and "google.protobuf.duration". Apache Beam uses int64 MAX and MIN as sentinel values for an +inf watermark and -inf watermark. However, the google.protobuf.timestamp is compliant with RFC3339, meaning it can only represent date-times between 0001-01-01 and 9999-12-31. This is not the same as the int64 MAX and MIN representation. The questions remain:

• What should the timestamp and duration representations be?
• What units should the timestamps and duration be? Nanos? Micros?
• What algebra is allowed when dealing with timestamps and durations? What is needed?

See:

• https://lists.apache.org/thread.html/c8e7d8dc7d94487fae23fa2b74ee61aec93c94abbcbef3329ffbf3bd@%3Cdev.beam.apache.org%3E
• https://lists.apache.org/thread.html/27fe9aa5b33dbee97db1bc924ee410048137e4fe97d9c79d131c010c@%3Cdev.beam.apache.org%3E