Fork Improvements (#624)

- Add a "was_forked_from" field to workflow status. This is true if the
workflow was ever forked from, and false otherwise. This can be filtered
on from list workflows. If you're forking workflows to recover from
failures, this lets you "filter out" the original executions and only
show the forked re-attempts.
- Add a "fork from failure" operation to Conductor that forks a workflow
from its last failed step (or last step, or from a specific step, or
last occurrence of a particular step name). Enables a quick "bulk fork"
action from the UI for the common case, though more complex cases will
still require client scripting.
- Add a "replacement children" option to fork that maps original child
workflow IDs to replacement child workflow IDs. When the forked workflow
encounters a step that started a child workflow matching an original ID,
it substitutes the replacement ID instead. This is useful when you need
to fork a parent workflow that depends on the results of child workflows
that have also been forked.