Client-side networking failure is never reported as error

Problem

• The settings page says "API keys have been verified and are correct" even when the client was not able to connect to Mollom.

Cause

• The NETWORK_ERROR case is not handled separately by _mollom_status().
• In case API keys were verified as correct before, the configuration status is not updated (to account for potential downtime) — this can mean that API keys were successfully verified on a local developer machine, but can't be verified on a production site (due to a firewall), which in turn causes the inappropriate positive status message to stick.

Proposed solution

1. Change _mollom_status() to use a cache with expiration date (instead of a variable) — re-verify once per day when API keys are valid, and once per hour in case of any kind of API key verification error.
2. Remove the fallback to the last known good configuration status (i.e., the downtime protection).
3. Ensure that the NETWORK_ERROR status code is retained.

Notes

• Some developers are force-disabling all caches during development, which will cause an API key verification call on every page request. This cannot be prevented.
• The current variable-based status is prone to a much more problematic misconfiguration: Some developers are using the Features/Variables module to control all variables in code. In that case, the configuration status variable might not be updated at all.