Enable caching of side-input dependent computations

Sometimes the kind of computations one wants to perform in startBundle depend on side inputs (and, implicitly, the window). For example, one might want to initialize a (non-serializable) stateful object. In particular, this leads to users incorrectly (in the case of triggered or non-globally-windowed side inputs) memoizing this computation in the first processElement call.

One option would be to fold this into a customizable ViewFn.

Imported from Jira [BEAM-1002](https://issues.apache.org/jira/browse/BEAM-1002). Original Jira may contain additional context.
Reported by: robertwb.

Start by tracing how startBundle receives side inputs and how the customizable ViewFn option fits the existing Beam model. Define caching semantics for triggered and non-globally-windowed side inputs, including when window-dependent state must be refreshed; the work is done when those computations are cached correctly without first-element memoization.

Written by the indexing model from the issue text.