bump leap.common min required version, new change needed
'collect_plugins'.

Tag version 0.7.0.

Conflicts:
	client/pkg/requirements.pip
	common/pkg/requirements.pip

Previous to this change, the actual encryption method used to run on its own
thread. When the close method was called from another thread, the queue could
be deleted after the encryption method loop had started, but before the queue
was checked for new items.

By removing that thread and moving the encryption loop to the reactor, that
race condition should disappear.

Closes: #7088.

- Related: #6359

Instead of opening one TCP connection for each HTTP request, we want to reuse
connections. Also, we need to be able to verify SSL certificates. This commit
implements both features in the twisted http client sync.

This change uses twisted deferreds for the whole syncing process and paves the
way to implementing other transport schemes. It removes a lot of threaded code
that used locks and was very difficult to maintain, and lets twisted to the
dirty work. Furthermore, all blocking network i/o is now handled
asynchronously by the twisted.

This commit removes the possibility of interrupting a sync, and we should
reimplement it using cancellable deferreds if we need it.