Enable connection keep-alive on RemoteGraphQLDataSources by default.

[abernix]

In Apollo Gateway, the default fetcher that the RemoteGraphQLDataSource uses doesn't configure a Node.js http.Agent with the keepAlive: true option. This means that the default behavior is to destroy the underlying TCP sockets when there are not active connections rather than maintaining a pool of established connections.

The current default fetcher in Apollo Gateway is node-fetch. Some alternative fetchers support having keep-alive on by default, but it's also possible to configure node-fetch to have this option enabled by default, as documented in their documentation.

We've found a number of reasons in the past to recommend make-fetch-happen — notably it's out of the box support for proxies, conditional cache control headers, gzip/deflate support, and its request pooling. We should:

• Evaluate whether it's feasible to change our default fetcher to make-fetch-happen, which solves this. This is a bit of a larger change and something we've punted before. This is actually tracked in Apollo Gateway: Add make-fetch-happen fetcher as the default fetcher for the downstream services #192, so if we take that approach (which has some comments with reading on it and its own associated references!), we may be able to just close this issue as resolved.
• If that approach is deemed too large, we may want to just keep the default fetcher as node-fetch but configure it to use keepAlive: true.