Add basic guards to avoid firing observer unnecessarily

For example, we don’t need to recalculate all queries when a QUERY_STOP action happens, and this can be easily detected by === the data field.

[stubailo]

Currently, broadcastQueries is called every time there is a change in the Redux store.

This then iterates over all of the query listeners, and calls those (defined here).

That ends up calling readSelectionSetFromStore if certain conditions are met, like if there are no errors, and the query is not loading, etc.

That calls observer.next, even if the query result is the same as before, which could end up re-rendering react components, etc.

The end result is that every time there is a Redux store change (even one that didn’t affect apollo at all), we re-run readSelectionSetFromStore and re-fire the next on any observable subscribers currently active.

However, the actual query results change pretty rarely! For example, if state.apollo === previousState.apollo, then we shouldn’t run anything at all. If state.apollo.data === previousState.apollo.data, then we don’t need to update any query results that we have already sent, etc.

This task is about adding some simple equality checks in the process to avoid firing every single query observable on every store change.

[abhiaiyer91]

Solved in #226