[#74523706] Add timings for workflow components

Push how long it takes the main workflow components
(read: goroutines) to complete per run to StatsD.

Also created a separate util function StatsDTiming that takes a
label, a start time and an end time and pushes the timing metric
through to StatsD.
Merge pull request #21 from alphagov/74523706-add-timings-to-workflow-components

[#74523706] Add timings for workflow components