Implement client-side OAuth in non-local HTTP connections. Also add tests to
verify that the different modes of connection are set up correctly.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=147636222
Move all of the code to create the request factories into
RequestFactoryModule. Also add the --force_http_connection flag to allow us
to force the use of HTTP connections instead of HTTPOverRPC for our internal
connections.
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Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=146116640