MissingDesignDocError raised on get_sync_info due to a missing design
document will be handled by the server during sync.
Ensure is now False by default, and thus database creation can deliver
an empty one that will be ensured during sync, following the ensure
parameter.

This error raises while getting info on target (or server) replica. On
previous implementation there was nothing to do here, but now that we
have db creation in place this error should be handled just like u1db
original implementation.
The reason is that db creation occurs during sync exchange, but before
this we try to ask for info and the code that checks for info raises an
error that will be used to signal the client that a database will be
created and that it must save the replica uid returned by server, after
sync exchange (where we send/fetch documents).

this is a workaroud to reduce the chances of failed sync due to
timeouts. this should be properly tackled by:

1. implementing proper cancellable for the sync operation.
2. implementing a retry count at the level of a single request, handled
internally by soledad.

in this way we can remove the retries logic from the soledadbootstrapper
in the bitmask client.

- Related: #7382

- Related: #7503

- 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