Current cql transport code acquire a permit before processing a query and
release it when the query gets a reply, but some quires leave work behind.
If the work is allowed to accumulate without any limit a server may
eventually run out of memory. To prevent that the permit system should
account for the background work as well. The patch is a first step in
this direction. It passes a permit down to storage proxy where it will
be later hold by background work.
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