When triggers are changed via pipeline update, stale finished triggers data applied

TriggerRunner tracks which trigger subexpresions are finished. When the trigger expession is updated via pipeline update, this data is applied arbitrarily to the new trigger expression.

Implementation note: trigger subexpressions are identified by number, and their finished state stored in a bit set.

Imported from Jira [BEAM-31](https://issues.apache.org/jira/browse/BEAM-31). Original Jira may contain additional context.
Reported by: kenn.

Start by locating TriggerRunner and the pipeline-update path that replaces trigger expressions. Inspect how trigger subexpressions are numbered and how their finished state is restored from the bit set. Done means finished state from the old expression is not applied arbitrarily to the updated expression, with behavior verified by appropriate regression coverage.

Written by the indexing model from the issue text.