Fix issues found when creating integration tests

[dubious90]

The resulting PR was too large, so splitting these fixes out from the full integration PR:

1. FakeUserDefinedOutputPlugin is now thread safe.
2. AggregateGlobalOutput takes in UserDefinedOutput so that it can account for errors

• Previously there was no way for the global aggregate to know about missing outputs which could have created confusion for users

3. FakeUserDefinedOutput - handleResponseData now ignores empty data responses

• We uncovered an issue where handleData gets called twice per request, once with empty data - more investigation required in In User Defined Output plugin framework, handleResponseData called twice per request #950

4. fake_user_defined_output plugin can no longer be test only because of a dependency chain that leads to non-test code, such as benchmarks and dynamic_config. It is too complex to break this dependency chain at the moment.

[dubious90]

@fei-deng please review and assign to @mum4k when ready

[fei-deng]

LGTM. Nice change dubious90.