HTTP client cached connections will hang around in the reactor if they are not
properly cleaned up, and might raise a "reactor unclean" message on shutdown.
This commit adds a close() method to the client http target that will cleanup
those connections.

after suggestions in the review

implementing a generic plugin interface to allow other modules to react
to soledad syncs, receiving a list of document ids that they've
subscribed to.

- Resolves: #6996
- Releases: 0.7.1

When async decrypting, we want to finish as fast as possible. When encrypting,
though, we don't have such a rush. With an encryption loop period of 2
seconds, we're able to encrypt 30 documents in one minute (the current bitmask
client sync period), which is meaningful: should moderatelly use the processor
while not syncing and relief from some work when actually syncing.

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.