replace msg_store:attrs with contains

This is a step on the path to getting rid of message attributes in msg_store.

msg_store:attrs was only being used in disk_queue:prune, to detect
when the store contained a non-persistent message and remove that
message from the store and the rabbit_disk_queue table.

Now rabbit_disk_queue records contain an IsPersistent flag. By making
the msg count delta generator pay attention to that flag we trim
non-persistent messages from the store during its initialisation,
disk_queue:prune no longer needs to remove messages from the store, it
just needs to remove all messages from the rabbit_disk_queue table
which are no longer referenced by the store - hence the new
msg_store:contains function.

Keeping the IsPersistent flag in the rabbit_disk_queue table is
sub-optimal since it means we store it once per message reference
rather than just once per message. That's a small price to pay though
for the cleaner interaction between the disk_queue and msg_store, and
the opportunity to remove the notion of message attributes from
msg_store altogether.

Populating the new field in rabbit_disk_queue is straightforward in
most places except disk_queue:tx_commit. That used to just be given
{MsgId, IsDelivered} tuples, so I had to change the API to {MsgId,
IsDelivered, IsPersistent} tuples.

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