Consider exposing task stopped state

While testing the cyclic tasks for https://github.com/hardbyte/python-can/pull/412 I noticed we don't expose whether a task is currently active or stopped. Specifically I was trying to see the state of `ThreadBasedCyclicSendTask` which has a `stopped` boolean attribute which isn't in the Task api and ASFAIK isn't thread safe.

If this was modified to be a property we could check an underlying driver/kernel for various backends to query if the task is active.