[SessionD] Clean up LocalEnforcer

[themarwhal]

Summary

4 things are covered in this PR:

1. Logic refactor on the session termination flow
   This change was brought up because I was noticing this log all the time when observing SessionD logs. [IMSI] Forcefully terminating session since it did not receive usage from pipelined in time. I initially thought there was a bug somewhere that was causing sessions to be terminated forcefully all the time. (As opposed to terminating itself after PipelineD's enforcement stats were cleared.) But it turns out the logs were inaccurate.
   This log comes from SessionState's complete_termination, which is used in both force termination and regular termination. So the fix here is to update the logs to reflect the behavior.
   Finally, while I was looking at the function, I noticed that LocalEnforcer passes the SessionReporter into the function, which is a bit of an anti-pattern since we do all reporting from either LocalSessionManagerHandler or LocalEnforcer. So I moved out the logic to LocalEnforcer.
2. Some cosmetic changes to rename variable update_criteria -> uc
3. Light refactor to remove unused parameters/return values from LocalEnforcer
   There are to main changes. Many of the LocalEnforcer functions had 'session_map' as a parameter when it is not used at all. (This is probably left over from legacy code). Second, PipelineDClient always returns true as a result of making an asynchronous call, so there is no value in casing on its return value.
4. test_local_enforcer clean up + deprecate test-only methods
   Remove LocalEnforcer's get_charging_credit and get_monitor_credit since they are only used for testing. The functions also just have a questionable way to go from IMSI, bucket -> value. (Sessions are unique by IMSI+SessionID, not just IMSI). Instead, assert_charging_credit and assert_monitor_credit are used in test_local_enforcer.cpp. Additionally, instead of having hardcoded imsi/sessionID test values, migrate to a model where we use a predefined set of values.

Test Plan

unit tests, s1ap test (ran it locally it passed)

Additional Information

• This change is backwards-breaking

[uri200]

lgtm