rust(bug): Improve how sift-stream handles the flow config cache

[tsift]

Improves how sift-stream handles the flow config cache internally.

• On initialization, if no flows are provided, sift-stream syncs with everything Sift has.
• On initialization, if flows are provided, sift-stream only syncs those with Sift.
• When using "add_new_flows", any flows that exist in the flow config cache are filtered out.

Note, the initialization behavior w/r/t Sift is unchanged, sift-stream simply no longer will throw away the resulting flow configs when none are provided during initialization.

These changes improve the restart behavior of sift-stream or restarts of upstream callers. For example, if an upstream entity is responsible for streaming flow "foo" and always attempts to add flow "foo" when starting, that entity disconnecting and reconnecting would previously always hit an "already exists" error for subsequent calls to "add_new_flow". The same scenario would occur if sift-stream itself needed to be recreated, since effectively the upstream entity would perform the same actions.