ConcurrentModificationException @ ApolloCacheInterceptor

AppSync SDK Version: 3.1.4 (latest as of Jul 7)

It's pretty straightforward. This should be all you need to fix but please let me know if you need anything.

**Stacktrace**
```
Fatal Exception: java.util.ConcurrentModificationException
at java.util.LinkedHashMap$LinkedHashIterator.nextNode(LinkedHashMap.java:757)
at java.util.LinkedHashMap$LinkedValueIterator.next(LinkedHashMap.java:785)
at com.apollographql.apollo.internal.interceptor.ApolloCacheInterceptor$2.apply(ApolloCacheInterceptor.java:137)
at com.apollographql.apollo.internal.interceptor.ApolloCacheInterceptor$2.apply(ApolloCacheInterceptor.java:134)
at com.apollographql.apollo.api.internal.Present.map(Present.java:70)
at com.apollographql.apollo.internal.interceptor.ApolloCacheInterceptor.cacheResponse(ApolloCacheInterceptor.java:133)
at com.apollographql.apollo.internal.interceptor.ApolloCacheInterceptor.access$200(ApolloCacheInterceptor.java:43)
at com.apollographql.apollo.internal.interceptor.ApolloCacheInterceptor$1$1.onResponse(ApolloCacheInterceptor.java:81)
at com.apollographql.apollo.internal.interceptor.ApolloParseInterceptor$1.onResponse(ApolloParseInterceptor.java:71)
at com.apollographql.apollo.internal.interceptor.ApolloServerInterceptor$1$1.onResponse(ApolloServerInterceptor.java:100)
at okhttp3.RealCall$AsyncCall.run(RealCall.kt:140)
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
at java.lang.Thread.run(Thread.java:923)
```

Start with ApolloCacheInterceptor.cacheResponse around line 137, identified in the stack trace, and inspect how the LinkedHashMap values are traversed. Reproduce the reported ConcurrentModificationException against AppSync SDK 3.1.4 if possible, then verify that the interceptor no longer throws during concurrent cache response handling.

Written by the indexing model from the issue text.