rlm_kafka: debug-build sanity check that dr / error cbs run on-thread

rlm_kafka: debug-build sanity check that dr / error cbs run on-thread

Capture pthread_self() into rlm_kafka_thread_t at thread_instantiate,
then assert the same tid in _kafka_delivery_report_cb and
_kafka_error_cb.  librdkafka only wakes us via the main queue (which
we poll from the worker's event loop), so a cross-thread hit would
mean an event slipped a different path and the no-lock handling of
the inflight list is unsafe.

Field and assertions are #ifndef NDEBUG so release builds carry
neither the extra tid nor the check - fr_assert(_x) expands to
nothing under NDEBUG so the missing field doesn't matter.