fix for wrongly ommiting FORK EVENT

When hived is stopped and then started to continue syncing blocks, then
it adds artificial BACK_FROM_FORK events, which forces applications
to clean up reversible data collected just before stoping the hived.
Attached applications which are traversing range of irreversible blocks
ommited fork events, because they have only irreversible data, but
servicing of fork event not only reverts reversible data, but also
change contexts fork_id, and in consequences when the applications
reached first new reversible block, then their views did not see it
because contexts have old fork_id.