- 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.

Preparing many docs is useful for batching only. As we are sendind one
by one I will leave prepare_one_doc method to do the encrypt as the docs
goes to upload.
Also fixes method name as kaliy suggested.

isinstance is better, as kaliy pointed out, and the constructor is also
in a safer place on __init__.py to be explicit.
Also re-apply a change from last rebase;