dev/core#1932 - Make status-checks more polite during upgrade

dev/core#1932 - Make status-checks more polite during upgrade

Before
------

If you happen to run a status-check (eg `showPeriodicAlerts()`) in the interim between
downloading code and running DB upgrades, then you'll get weird failures.

After
-----

The status-checks are able to complete.

Technical Details
-----------------

(1) I figure that this conditional is lightweight because it relies on data
that's read-once and then cached in memory:

```
CRM_Utils_System::version() !== CRM_Core_BAO_Domain::version()
```

(2) The easiest way to reproduce is to get a DB from before 4.7 and coerce
`CRM_Utils_Check::CHECK_TIMER`.