Posix timestamps for logs (updated title)

[See original issue on GitLab](https://gitlab.com/BuildStream/buildstream/-/issues/892)
In GitLab by [[Gitlab user @willsalmon]](https://gitlab.com/willsalmon) on Feb 1, 2019, 15:35

## Background

The logs do not have as many options as may be useful for analysing them.

They should has a more precise wall clock option, (in IRC conversations we thought we had this)
we should also have something that is approximately a standard UNIX timestamp.

We would like it to be easy to configure the logs so that humans and computers can effectively parse the logs.

## Task description

[//]: # (Short summary of the action to be executed)

* [x] wallclock-us option
* [ ] timestamp
* [x] Verify that at least one of these can be quickly read by a computer so that parsing the log of something like the Debian build project is not expensive, and if not add a more easily machine readable option.

## Acceptance Criteria

This will be complete when it is easy to get the exact time a event happened in the logs in such a way that is not too expensive for a computer or human to read (this may require two thing being printed, one for he human and one for the computer, if two are needed it the computer readable should not inter fear with reading/parsing/displaying the logs, eg. base64 or parsing integers rather than floats etc.)

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Start by reviewing the existing wallclock-us log option and the original GitLab issue linked in the description, along with the discussion comments. Determine how a timestamp option should be configured and represented so humans and computers can read it efficiently. Done means logs expose the exact event time in a precise, machine-readable form without making normal log display difficult to parse.

Written by the indexing model from the issue text.