Increasing max sockets to 20

#### What's this PR do?
Increases the number of concurrent sockets per host to 20. Pretty much all requests this app fields will be from Mobile Commons, so I think it'd make sense that we could hit our limit with particularly high volume.

Express uses the `http` module, so setting the global agent setting here works.

#### How should this be manually tested?
I tested with `$ npm test` and POSTing to `ds-routing/tips`.

#### What are the relevant tickets?
Closes #339