Client refactor WIP
Add facility to wait for a certain number of messages to DeviceTestServer
WIP for an ioloop manager that can be shared between client and server implementations
Create a new future in handle_message.

Need to create a new future before returning it to KATCPserver, or it will also
get the exceptions handled by send_reply()
multi-stopping fixup
Reduce logging noise by having mass_send_messages skip closed connections
Improve test reliability by increasing silly short timeout
Merge pull request #26 from ska-sa/factor-ioloop-manager

Factor ioloop manager