hfm: lock hive.contetxs when hived removes events

There was a race condition between attaching a context
and removing events by the hived:
- during attaching a context it's event id is changed from
  unreachabe_event_id to 0, to block removing events before
  a new event id will be found for a context. Immediatly after
  attach find next event is executed, and some event id is found

- in the meantime hived may remove uneccessary events based on
  context id, which at the beginning was ureachable_event_id, so
  all the events will be removed

Finally context got an event id which is being removed by hived, and
in consequence will violent FK between contexts and events.

The solution is make a lock between attaching a context and removing
events.