Call clearStore callbacks after clearing store

[ds8k]

This makes clearStore work similarly to how resetStore works, which is to say the cache is first cleared and then callbacks are run. This resolves issue #4694

[apollo-cla]

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[benjamn]

Thanks @ds8k! I made some simplifications that are possible now that we do not lazily create client.queryManager, but the functional change you proposed seems like a good idea.