Includes a "blocking" flag on record creation.

This change allows you to use caput in asynchronous mode, where it will
wait for record processing to complete.

This implementation still leaks abstractions - the device now needs to
know about the difference between cothread and async calls.
Perhaps another round of refactoring is required...

Also add single and multi threaded tests for blocking records.

Renamed the function to conform to naming convention.
Added CHANGELOG entry.

This leaks even more async code into the device, but I can't see a way
around it without having to require the dispatcher be provided before
creating records...

By creating/modifying the dispatchers we can easily handle the
__completion being called after the __on_update.
Without doing this there seem to be unavoidable implementation leaks,
where the device.py file would have to care about the difference
between cothread and asyncio.

- SetBlocking returns the old blocking state value. Added test.
- Fix some style issues
- Make completion function on the dispatcher optional, to maintain some
compatibility with the previous API