P2P use round trip of time_message to calculate latency

[heifner]

Address peer review comment: #1215 (comment)

Use round trip time for time_message which was already implemented. Since this already did a round trip for previous versions of P2P, no new P2P protocol version is needed. All that was needed was to add the calculation and normalize epoch time. Previous versions of leap did not specify duration resolution of time_since_epoch. Some clients returned microsecond values and others returned nanosecond values. Included in this PR is a fix so that all nodeos going forward will return nanosecond values even if the resolution is only microseconds.

Resolves #1072

[greg7mdp]

Kevin, could you improve the comments on these?

      tstamp                         org{0};          //!< originate timestamp
      tstamp                         rec{0};          //!< receive timestamp
      tstamp                         dst{0};          //!< destination timestamp
      tstamp                         xmt{0};          //!< transmit timestamp

[heifner]

Replaced by #1243. Github somehow became out of sync with this branch. Pushing to the branch did nothing.