Refactor Cipher related things.

Refactor Cipher related things.

Instead of using a hash with set keys, set the key on the relevant
  CoreIrc{User,Channel}'s cipher member.
Let the cipher clear its own key (with appropriate checks elsewhere)
Instantiate a new IrcUser when necessary (a query with someone who does
  not share any channels with us means there is no IrcUser for the target
  until he says something)