Seperate service+client from threads.go

[sanderpick]

From @jsign: Make all exported methods in threads.go thread-safe

See https://github.com/textileio/go-textile-threads/pull/133#discussion_r351804962

[jsign]

I have the feeling that maybe I left this invariant hold now, but not 100%. Good for checking again, and for future APIs (if they mutate things in thread (or its logs)).