Fix counting number of triggers again

Sorry, 8a8dc80a doesn't make sense. I misunderstood behaviour of
arg.useDistinct. Now I use a string which includes server ID &
trigger ID to distinguish duplicated records because count()
function doesn't accept multiple arguments.
    
Although there are some ways to use multiple columns to distinguish
duplicated records on counting number of rows, I think it is the
most simple solution at this time. On the other hand I think using
sub query is the formal solution (github issue #168), so this code
will be removed after the sub query solution is introduced.