Group queue deletions on_node_down into 10 operations per transaction

When many queues are being deleted, we believe that it's faster to have
fewer Mnesia transactions and therefore group 10 queue deletions into a
single Mnesia transaction. This number (10) is arbitrary, we didn't try
with a different number. Creating 1 Mnesia transaction for every queue
deletion feels too many transaction, and having a single Mnesia
transaction for all queue deletions is too few transactions. This felt
like a sensible option.

We cannot determine if this is a good change because
rabbit_core_metrics:queue_deleted/1 takes the most time and obscures all
observations. According to qcachegrind,
rabbit_misc:execute_mnesia_transaction/1 takes 1.8s while
rabbit_core_metrics:queue_deleted/1 takes 132s out of which ets:select/2
takes 131s.

How can we optimise rabbit_core_metrics:queue_deleted/1 ? We are
thinking that rather than calling ets:select/2 twice for every queue, we
should call it twice for all queues that need to be deleted. We don't
know whether this is possible. Alternatively, we might look into
ets:first/1 & ets:next/2 to iterate over the entire table ONCE with all
the queues that have been deleted. Thoughts @dcorbacho @michaelklishin ?

For initial context, see #1513

Partner-in-crime: @essen

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