Add an option to make optional the possibility to store diagnostics timestamps

[bourtemb]

In the current version, the library is storing the event receive time and event insert time into the database.
It would be useful to add an option to be able to enable this feature only on-demand.
It appears that these timestamps do not seem to be very useful since the EventSubscriber is already monitoring the min and max processing time. Disabling this feature could help saving quite a lot of space.

[chedburgh]

This will be included in a much larger change set I will add as a branch shortly. I will outline the code that adds this change here.

[chedburgh]

Included in branch Refactor-Development

[chedburgh]

Config variable store_diag_times was added. Default is true, to maintain like behaviour with the current module. When set to false, it will stop storing diagtimes. Code can be traced with this flag.

Intention is to move this to default false in the future.