proposer: wait for a hash to be in the active-leaves set (#1616)

* overseer: add ExternalRequest to Event

* proposer: wait for the hash to be activated

* update comments

* overseer: handle unbounded growth of listeners map

* overseer: fix compilation

* overseer: clean up dead listeners

* overseer: cosmetic changes

* overseer: cosmetic changes t.2

* overseer: add debug_assertions

* overseer: fix formatting