(Placeholder for bug link so I can put Phrabricator number in commit.)
I previously submitted a patch in the past, but it was rejected for fixing the issue.
* This new patch adds a AbuseFilterCentralDb using an Extension:Echo like method of centralizing where the central/global database connection is handled.
* This allows connections using $wgExternalServers to work with the new setting $wgAbuseFilterCentralCluster.
* It is also backwards compatible so that if only $wgAbuseFilterCentralDB is configured it will use the outdated wfGetDb() method.
* Many places that were reusing global'ing $wgAbuseFilterCentralDB in and checking it have been simplified.
* Database connection and AbuseFilterCentralDb instance reuse are enforced as needed.