rec: drop ref in mtasker when it is no longer needed

[omoerbeek]

Short description

If we do not do that, only the next call of sendEvent will release
the shared pointer to the TCPConnectionHandler in PacketID as it overwrites d_eventkey. Same for d_waitval. This
might delay cleaning TCP objects up on not-so-busy threads.

Fixes #13422, thought it can still take a while on idle recursors,
as the housekeeping function is not called often in those cases.

Added a simple mechanism to make sure d_eventkey is not used when it is not set yet or after being moved.

Checklist

I have:

• read the CONTRIBUTING.md document
• compiled this code
• tested this code
• included documentation (including possible behaviour changes)
• documented the code
• added or modified regression test(s)
• added or modified unit test(s)

[coveralls]

Pull Request Test Coverage Report for Build 11665554436

Warning: This coverage report may be inaccurate.

This pull request's base commit is no longer the HEAD commit of its target branch. This means it includes changes from outside the original pull request, including, potentially, unrelated coverage changes.

• For more information on this, see Tracking coverage changes with pull request builds.
• To avoid this issue with future PRs, see these Recommended CI Configurations.
• For a quick fix, rebase this PR at GitHub. Your next report should be accurate.

Details

• 62 of 67 (92.54%) changed or added relevant lines in 2 files are covered.
• 35 unchanged lines in 11 files lost coverage.
• Overall coverage increased (+0.04%) to 64.661%

---

Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
pdns/recursordist/mtasker.hh	9	10	90.0%
pdns/recursordist/test-mtasker.cc	53	57	92.98%

Files with Coverage Reduction	New Missed Lines	%
pdns/recursordist/mtasker.hh	1	88.19%
pdns/tcpiohandler.cc	2	68.2%
pdns/rcpgenerator.cc	2	89.68%
pdns/dnsdistdist/dnsdist-async.cc	2	82.11%
pdns/recursordist/test-syncres_cc2.cc	3	88.85%
pdns/communicator.cc	3	71.85%
pdns/recursordist/syncres.cc	3	79.43%
pdns/communicator.hh	3	77.78%
pdns/opensslsigners.cc	3	61.41%
pdns/dnsdistdist/dnsdist-carbon.cc	6	62.16%

Totals
Change from base Build 11665506881:	0.04%
Covered Lines:	124881
Relevant Lines:	162317

---

💛 - Coveralls

[rgacogne]

Looks good to me!
If it's not a lot of work (I have no idea how hard it would be), adding a unit test to ensure that we do not re-introduce the same issue in the future might be a good idea.