replicate-rewrite-db additions at runtime for multi-source replication ease of use

Currently, replicate-rewrite-db-variables have to be set at server startup (either via command-line or via configuration file). This works, but gets really annoying when combined with multi-source replication.

Consider the case where one MariaDB instance is used as a replication sink for multiple other databases when said databases have the same name (for the sake of argument, let that name be dbname).

In order to avoid collisions on the single replication slave and its dozen master connections, each one has to have replicate-rewrite-db set to something sensible (say master1.replicate-rewrite-db=dbname->dbname_master1, master2.replicate-rewrite-db=dbname->dbname_master2, etc.).

When adding the next database to this setup, a server restart of the multi-source slave is required. This can be a painful process considering that the slave may have long-running queries or processes which cannot easily be interrupted (for instance, adding a few terabytes of mysql dumps from further databases to be replicated – during the time said dumps are being replayed, effectively no further replication slaves can be added).

It would increase quality of life/ease of use a lot if the replicate-rewrite-db-variables were settable/changeable at runtime, thus this feature request.