Pass on broker information to flush callback for better debugging

If you have setup

    {ekaf,[
        {ekaf_callback_flush, {mystats, callback_flush}}
        %% where the callback is fun mystats:callback_flush/5
    ]}.

then change mystats.erl from

    callback_flush(Topic, Partition, BuffLen, Worker, CorId)->

to

    callback_flush(Topic, Broker, Partition, BuffLen, Worker, CorId)->

Bumping version to 1.2