Provide client in activity context

[dandavison]

Fixes #721

[dandavison]

This needs a bit more work -- as things stand it's attempting to pickle the client in the context in tests of process-based executor pools (which fails when it tries to pickle the RuntimeRef), but I wonder if that means it is attempting to share the client across threads in thread-based executor pools.

[cretz]

I'd be ok for now raising an exception for if the activity.client() is invoked in def activites (so async def activities only for now)

[dandavison]

Rebased and ready for review again.

Makes the worker's connected client.Client available in activities as activity.client().

As discussed above, the client is only available in async def activities.