Smok-client

* fixed closing `Event`s
* added pickling `Event` and `Macro`
* `on_read` Future can now return `None`

* added `Pathpoint.get_archive`
* definitively removed set metadata
* added custom `SMOKDevice.execute_section`

* added setting and reading linkstate and instrumentation metadata for slave devices
* added a true macro database

* added pluggable pathpoint value databases
* `Section` is now a `Future`
* **bugfix**: timestamp from restored data would be needlessly bumped up
* added option to create `Events`

* minor refactorings and docs improvements
* added `sensors` and `predicates` for device