- bug: we were dumping the received secrets locally to disk *before*
  setting the received property for the active secret, and therefore the
  'active_secret' was always marked as null.
- refactor common code into an utility method.

We are getting "too many files open" while running tests with 1024 max
files open. This is a leak from close methods. Some of them should be fixed
on this commit, but further investigation may be necessary.

in this way we use the reactor pattern to dispatch the events, instead
of having the overhead of running a separate client thread.

- Resolves: #7274

Code is trying to close a closed threadpool. This raises errors on
Twisted 15.4.

- Resolves: #7412

The http_target.py refactor started in 8adf2dedb74941352520d8de42326b0c59818728
forgot to remove the old file.