dnsdist: Add DynBlockRulesGroup

dnsdist: Add DynBlockRulesGroup

The new `DynBlockRulesGroup` object is designed to make the processing
of multiple rate-limiting rules faster by walking the query and
response buffers only once for each invocation, instead of once per
existing `exceed*()` invocation.

For example, instead of having something like:
```
function maintenance()
  addDynBlocks(exceedQRate(30, 10), "Exceeded query rate", 60)
  addDynBlocks(exceedNXDOMAINs(20, 10), "Exceeded NXD rate", 60)
  addDynBlocks(exceedServFails(20, 10), "Exceeded ServFail rate", 60)
  addDynBlocks(exceedQTypeRate(dnsdist.ANY, 5, 10), "Exceeded ANY rate", 60)
  addDynBlocks(exceedRespByterate(1000000, 10), "Exceeded resp BW rate", 60)
end
```

The new syntax would be:
```
local dbr = dynBlockRulesGroup()
dbr:setQueryRate(30, 10, "Exceeded query rate", 60)
dbr:setRCodeRate(dnsdist.NXDOMAIN, 20, 10, "Exceeded NXD rate", 60)
dbr:setRCodeRate(dnsdist.SERVFAIL, 20, 10, "Exceeded ServFail rate", 60)
dbr:setQTypeRate(dnsdist.ANY, 5, 10, "Exceeded ANY rate", 60)
dbr:setResponseByteRate(10000, 10, "Exceeded resp BW rate", 60)

function maintenance()
  dbr:apply()
end
```

The old syntax would walk the query buffer 2 times and the response
one 3 times, while the new syntax does it only once for each.
It also reuse the same internal table to keep track of the source
IPs, reducing the CPU usage.