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

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](https://issues.apache.org/jira/browse/BEAM-702). Original Jira may contain additional context.
Reported by: jkff.

Start by reading the DoFn lifecycle methods named in the issue: startBundle(), finishBundle(), setup(), and teardown(). Review Guava's Closer design for the proposed resource-management pattern, then determine the API and lifecycle behavior needed for automatic closing; done means bundle- and DoFn-scoped Closeable resources are reliably closed without user-written cleanup methods.

Written by the indexing model from the issue text.