dnsdist: add elapsed time to dq object

[phonedph1]

Short description

FFI seems to have this already.

Not sure if there is a reason not to include it in the normal Lua bindings?

Use case: want to easily check how much time has been spent so far, to decide if query should be replayed or not.

function result(dq)
        warnlog("query elapsed time is " .. dq:getElapsedUs() .. " us")

        return DNSResponseAction.None
end

addAction(AllRule(), LuaAction(result))
addResponseAction(AllRule(),LuaResponseAction(result))

addLocal("127.0.0.1:5454")
newServer({address="8.8.8.8"})

query elapsed time is 69.56 us
query elapsed time is 34759.856 us

Checklist

I have:

• [x] read the CONTRIBUTING.md document
• [x] compiled this code
• [x] tested this code
• [x] included documentation (including possible behaviour changes)
• [ ] documented the code
• [ ] added or modified regression test(s)
• [ ] added or modified unit test(s)
• [ ] checked that this code was merged to master

[coveralls]

Pull Request Test Coverage Report for Build 11943315686

Details

• 2 of 2 (100.0%) changed or added relevant lines in 1 file are covered.
• 5592 unchanged lines in 79 files lost coverage.
• Overall coverage decreased (-0.1%) to 64.632%

---

Files with Coverage Reduction	New Missed Lines	%
pdns/webserver.hh	1	69.16%
pdns/recursordist/sortlist.hh	1	75.0%
pdns/iputils.hh	1	78.2%
pdns/dnsdistdist/dnsdist-crypto.cc	1	76.01%
pdns/pollmplexer.cc	1	83.66%
pdns/dnstap.cc	2	70.97%
ext/probds/murmur3.cc	2	94.12%
pdns/dnsname.hh	2	91.84%
modules/godbcbackend/sodbc.cc	2	70.8%
pdns/sstuff.hh	2	56.83%
<!--	Total:	5592		-->

Totals
Change from base Build 11915618754:	-0.1%
Covered Lines:	125729
Relevant Lines:	163620

---

💛 - Coveralls

[rgacogne]

Marking this PR as a candidate for a backport to 1.9.x since it is very self-contained and could be useful there.

[phonedph1]

moved the docs up...