[FEATURE] Trade: Display counter for orderbook age (RT-2330)

I think the extra line
"Status: Current with latest ledger" is redundant with the display for
the orderbook age.

The age seems to follow a Poisson process with an average of 5s.
I have even observed the counter goes up to 60s before it resets.

The orderbook could be set to force-update after 30s timeout.
This likely solves the class of problems associated with orderbook
update.
$scope.lastUpdate = seconds ? seconds : 0;
=
$scope.lastUpdate = seconds || 0;