Properly check for FlowRef before touching caches

Properly check for FlowRef before touching caches

FlowRef is not necessarily present, hence we need to properly
isolate code that requires it.

This ends up being a larger-scale refactor, as there was a ton
of duplicate code. At the end of the day, this debugging facility's
footprint is reasonably reduced.

JIRA: OPNFLWPLUG-1106
Change-Id: I402cc302ab09e22bda7f731f7da4431f576c8bbb
Signed-off-by: Robert Varga <robert.varga@pantheon.tech>