Infer connectToDevTools regardless of whether another client has connected or not

[jerelmiller]

The connectToDevTools option currently sets a default value based on whether __DEV__ is set and whether another client has already connected to devtools (by checking window.__APOLLO_CLIENT__). While this worked well for the old devtools registration mechanism, we now have the ability to register multiple clients with the new devtools registration mechanism. We can and should remove the check for window.__APOLLO_CLIENT__ to allow connectToDevTools to be inferred based on __DEV__, otherwise it presents confusion when the first client is automatically registered, but subsequent ones are not. To workaround this currently, you need to explicitly set connectToDevTools to true in order for other created clients to be registered.

Work is still underway to actually be able to switch between multiple clients in the devtools. This work should be accomplished some time after that.