Replace logging.getLogger with custom get_logger.
Remove some unneeded dependencies and reorder other.

The backend was unsetting the soledad proxied object before actually calling
the soledad close() method and this was causing the application to hang when
exitting.

As the local soledad component's cancel_bootstrap() method is called before
the close() method, we'd better not unset the proxied object there, and let
only the close method do it. This commit fixes this by just removing the line
that unsets the proxied object in the wrong place.

Before this commit, the backend used plain pyzmq bindings for communicating
with the frontend. This implements the txzmq twisted-powered bindings instead.

Closes: #6360

... on them, to user-writeable only.

I think we could consider ipc for the curve-case too, at least for the
platforms that support them.

Use global flag for ZMQ_HAS_CURVE.
Closes #6646

This closes: #6658 and #6691

we should deal with this with pure deferreds