mirror: read from EAV tables by default (#1344)

Summary:
This flips the switch for all production `Mirror` reads to use the
single `primitives` EAV table as their source of truth, rather than the
legacy type-specific primitives tables. For context and design
discussion, see issue #1313 and commits adjacent to this one.

Test Plan:
All relevant code paths are already tested (see test plans of commits
adjacent to this one). Running `yarn test --full` passes.

wchargin-branch: mirror-eav-flip

src/graphql/mirror.js

@@ -1646,7 +1646,7 @@ export class Mirror {
     options?: {|+useEavPrimitives?: boolean|}
   ): mixed {
     const fullOptions = {
-      ...{useEavPrimitives: false},
+      ...{useEavPrimitives: true},
       ...(options || {}),
     };
     const db = this._db;