update context fork id when processing irreversible range

Processing back_from_fork event change context's current block num,
it was not a problem when an application process events before
reaching fork event, because block num was updated during servicing each
event. The problem is when some events before fork event  were removed
and the application is moving on irreversible blocks, in such situation
servicing fork event will change current blocknum regardles on which
irreversible block context is. Separated from fork processing update
contexts fork id was added.