It was expensive to delete files because we had clean up
the index and to get the messages in the file we have to
scan it.

Instead of cleaning up the index on file delete this
commit deletes from the index as soon as possible.
There are two scenarios: messages that are removed
from the current write file, and messages that are
removed from other files. In the latter case, we
can just delete the index entry on remove. For messages
in the current write file, we want to keep the entry
in case fanout is used, because we don't want to write
the fanout message multiple times if we can avoid it.
So we keep track of removes in the current write file
and do a cleanup of these entries on file roll over.

Compared to the previous implementation we will no
longer increase the ref_count of messages that are
not in the current write file, meaning we may do more
writes in fanout scenarios. But at the same time the
file delete operation is much cheaper.

Additionally, we prioritise delete calls in rabbit_msg_store_gc.
Without that change, if the compaction was lagging behind,
we could have file deletion requests queued behind many compaction
requests, leading to many unnecessary compactions of files
that could already be deleted.