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/c8e7d8dc7d94487fae23fa2b74ee61aec93c94abbcbef3329ffbf3bd@%3Cdev.beam.apache.org%3E)
* [https://lists.apache.org/thread.html/27fe9aa5b33dbee97db1bc924ee410048137e4fe97d9c79d131c010c@%3Cdev.beam.apache.org%3E](https://lists.apache.org/thread.html/27fe9aa5b33dbee97db1bc924ee410048137e4fe97d9c79d131c010c@%3Cdev.beam.apache.org%3E)

Imported from Jira [BEAM-8738](https://issues.apache.org/jira/browse/BEAM-8738). Original Jira may contain additional context.
Reported by: rohdesam.

Begin with Apache Beam's proto timestamp and duration representations, then review the linked mailing-list discussions and BEAM-8738 context. Done means the timestamp and duration formats, units, sentinel handling, and permitted algebra have a decided specification and clear implementation scope.

Written by the indexing model from the issue text.