OOIION-631 fixed.

Completed adjustments to fix https://jira.oceanobservatories.org/tasks/browse/OOIION-631 (along with some related code clean-up and internal refactoring).  All platform agent related unit and int tests passing.

**Note:** I have reviewed and adjusted all uses of the platform agent in coi-services according to this change. Not sure where other dependent code is located. The observation is that any client code should use ION system time(*) for the "from_time" parameter in the GET_RESOURCE command to the platform agent (instead of NTP time).

(*) compatible with the description of get_ion_ts function, https://github.com/ooici/pyon/blob/6a9e4199db1e9/pyon/util/containers.py#L243-248)