Refactor our logic for sending events to controllers

Previously we'd put these strings right on the controllers'
outbufs. But this could cause some trouble, for these reasons:

  1) Calling the network stack directly here would make a huge portion
     of our networking code (from which so much of the rest of Tor is
     reachable) reachable from everything that potentially generated
     controller events.

  2) Since _some_ events (EVENT_ERR for instance) would cause us to
     call connection_flush(), every control_event_* function would
     appear to be able to reach even _more_ of the network stack in
     our cllgraph.

  3) Every time we generated an event, we'd have to walk the whole
     connection list, which isn't exactly fast.

This is an attempt to break down the "blob" described in
http://archives.seul.org/tor/dev/Mar-2015/msg00197.html -- the set of
functions from which nearly all the other functions in Tor are
reachable.

Closes ticket 16695.