StateNamespaces.window() potentially unstable / stable encodings for windows

WindowNamespace#stringKey includes a base64 encoding of the window as encoded by the windowCoder. This makes the key unpredictable if we support features such as reloading a pipeline with a new SDK or bugfixed pipeline. The encoding should be stable and documented.

And even if the SDK's window types are stable and documented, users can define their own windows with coders and these may be less stable / requiring bugfixes. This ticket is to track the issue and any new ideas.

[1] https://github.com/apache/incubator-beam/blob/master/sdks/java/core/src/main/java/org/apache/beam/sdk/util/state/StateNamespaces.java#L113

Imported from Jira [BEAM-652](https://issues.apache.org/jira/browse/BEAM-652). Original Jira may contain additional context.
Reported by: kenn.

Start with sdks/java/core/src/main/java/org/apache/beam/sdk/util/state/StateNamespaces.java around line 113, where WindowNamespace#stringKey encodes the window with windowCoder. Investigate how stable encoding should work across SDK changes and user-defined window coders. Done means a concrete approach for stable encoding is established and documented.

Written by the indexing model from the issue text.