MT#4369 optimize callflow using a custom query

we need to disable the sorting option on the columns though

(cherry picked from commit 720e70b9397857565dc78766810c385164a5b813)

Conflicts:
	lib/NGCP/Panel/Controller/Callflow.pm
MT#4369 disable sorting on callflow page

as it has no effect, because of our optimized query

(cherry picked from commit a6ae6b1a84c8beab9e3e19b43f3762d860c3a09c)
MT#4369 use min_timestamp

to really display the time, the call started

(cherry picked from commit 83fa4552f264235a86cd6aa5769fc38f746ee163)

Conflicts:
	lib/NGCP/Panel/Controller/Callflow.pm
MT#4369 callflow: count total correctly

this group_by is ok, because it is optimized by mysql and the index on
call_id is still used

(cherry picked from commit c038691fea13150d7123c6758840aab02eadb27e)