Check structural equality in QueryInfo#setDiff, again.

Revisiting PR #6891, which was reverted by commit c2ef68f.

With the introduction of canonical cache results (#7439), the strict
equality check in setDiff is about to become mostly synonymous with deep
equality checking (but much faster to check), so we need to deal with the
consequences of #6891 one way or another.

As evidence that this change now (after #7439) has no observable impact,
notice that we were able to switch back to using !equal(a, b) without
needing to fix/update any failing tests, compared to the few tests that
needed updating in #6891.

However, by switching to deep equality checking, we allow ourselves the
option to experiment with disabling (or periodically clearing) the
ObjectCanon, which would presumably lead to broadcasting more !== but
deeply equal results to cache.watch watchers.

With deep equality checking in setDiff, those !== results will get past
the filter in the same cases canonical objects would, so there's less of a
discrepancy in client behavior with/without canonization enabled.

src/core/QueryInfo.ts

@@ -175,7 +175,8 @@ export class QueryInfo {
     const oldDiff = this.diff;
     this.diff = diff;
     if (!this.dirty &&
-        (diff && diff.result) !== (oldDiff && oldDiff.result)) {
+        !equal(oldDiff && oldDiff.result,
+               diff && diff.result)) {
       this.dirty = true;
       if (!this.notifyTimeout) {
         this.notifyTimeout = setTimeout(() => this.notify(), 0);