[mbox_indexer] Remove records from exludeMessageIdList after they are used

[lukas-vlcek]

When exclude list is provided we have to parse message in the "main" thread before it is handed to parallel thread for other processing (this is because we need to keep counter in synch with processed emails).

However, assuming each Message-Id is unique within mbox archive we can remove records from exclude list once they are used. This means that once all records from exclude list are used the list become empty and we could switch to more performant threaded processing.

Related issue #12