Simple pattern for per-bundle and per-DoFn Closeable resources

[damccorm]

Dealing with Closeable resources inside a processElement call is easy: simply use try-with-resources.

However, bundle- or DoFn-scoped resources, such as long-lived database connections, are less convenient to deal with: you have to open them in startBundle and conditionally close in finishBundle (likewise setup/teardown), taking special care if there's multiple resources to close all of them.

Perhaps we should provide something like Guava's Closer to DoFn's https://github.com/google/guava/wiki/ClosingResourcesExplained. Ideally, the user would need to only write a startBundle() or setup() method, but not write finishBundle() or teardown() - resources would be closed automatically.

Imported from Jira BEAM-702. Original Jira may contain additional context.
Reported by: jkff.