CRM-20958 - Extract helper Civi\Core\SqlTrigger\TimestampTriggers

CRM-20958 - Extract helper Civi\Core\SqlTrigger\TimestampTriggers

== Before ==

 * SQL triggers to populate `civicrm_contact.created_date` and  `civicrm_contact.modified_date` are
   generate via `CRM_Contact_BAO_Contact::triggerInfo($info, $tableName)`

== After ==

 * `CRM_Contact_BAO_Contact::triggerInfo` calls a helper `TimestampTriggers`
 * The helper `TimestampTriggers` accepts arguments describing the names of the tables/columns
   which needed for the timestamp triggers.

== Comments ==

To test, I used this command to update and dump the schema:

```
cv api system.flush triggers=1 && mysqldump --triggers ...
```

The schema was identical before and after.  (Notably, by alternately hacking
the code, I was able to validate the test was capable of revealing
discrepencies.)