Deferrable setting

[jackfischer]

Especially in data viewer but occasionally throughout other places as well we see a lot of inconsistent hanging that we are assuming comes from retrying serialization conflicts. We definitely don't want UI usage to make this worse especially in our production instance, so it would be great to be able to toggle on a setting to run the read only queries in read only + deferrable mode. If this were available we would plan to basically leave it on all the time and have everything except any mutation queries run as deferrable.