Require a recent (48h) positive check result

To the existing gate trigger filters, add the additional requirement
that the change must have a positive check result from within the
last 48 hours.  This should generally be automatically seen to at
this point by the preceding change which adds the check trigger.

Add an additional gate trigger for any verified column (also with
the 48h positive check result requirement).  This means that if a
user approves a change without a recent check vote, the check queue
will pick it up and provide one.  If it is positive, this new filter
will match on the gate queue and enqueue the change immediately
after the check result is reported.  It is likely to false-trigger
a lot on check results even before approval, however, the existing
checks of gerrit can-merge criteria will prevent it from adding such
changes to the queue.

NB: A reverify or reapproval immediately after a verified:-2 will no
longer work as that does not satisfy the verified:1 requirement.  An
intermediate check run will be required.  We can add an additional
filter that accepts recent verified:-2 votes if we wish.

Change-Id: Ie590d6a1feaa24bbc6264431e968f428dd4163a7
Merge "Require a recent (48h) positive check result"