- use subprocess.check_call() to ensure any errors during twistd startup
  will properly show up on test reports.

- use SIGTERM instead of SIGKILL to gracefully terminate twistd.

We were previously not using an empty local db for download benchmark
tests, so there was actually nothing to sync. This commit fixes that by
adding a way to force an empty local db on soledad client instantiation.

It needs a defer.Deferred

- Resolves: #8777

In order to be able to change passphrase, token and offline status of
soledad from the bitmask client api, the secrets api also has to be able
to use up-to-date values when encrypting/decrypting secrets and
uploading/downloading them to the server. This commit makes public some
soledad attributes that were previously "private" (i.e. used to start
with "_" and were not meant to be accessed from outside), and passes the
whole soledad object to the client secrets api. This makes the code
cleaner and also allows for always getting newest values of soledad
attributes.