Crash briefly observed in ut-extended when stopping HTTP connection pool

Search for "core dumped" at https://jenkins.aws.isc.org/job/kea-dev/job/ut-extended/1540/execution/node/1104/log/?consoleFull.

Occured one time on a single system. Probably due to a previous test not properly cleaning up.

Seems to happen on some sort of conflict on resource allocation or deallocation, maybe socket.

ASAN and TSAN warnings can be replicated relatively easily with two concurrent make checks:

export GTEST_FILTER='*.controlChannelShutdown'
make check -C src/bin/dhcp6 & make check -C src/bin/dhcp6

More details including ASAN and TSAN stacks on https://pad.isc.org/p/kea-unit-test-crash-2024-09-20.