FORCE mode for PURGE BINARY LOGS

Currently when requesting to PURGE BINARY LOGS up to a certain point the purge will stop early when reaching a binlog file that a slave is still reading from.

There can be situations where purging binlogs to recover file system space may be more important though than to not break slaves by pulling away binlogs from under their feet.

So there should be FORCE option for such cases that just disables the check for still active binlog files that slaves still fetch from.