Add periodic warning timers (every 5 minutes) to help diagnose hangs in
barrier_and_drain:
- drain_closing_sessions(): warn if semaphore acquisition or session gate
close is taking too long, reporting the gate count to show how many
guards are still alive.
- local_topology_barrier(): warn if stale_versions_in_use() is taking
too long, reporting the current stale version trackers.
- session::gate_count(): new public accessor for diagnostic purposes.
These warnings help distinguish between the two possible hang points
in barrier_and_drain (stale versions vs session drain) and provide
ongoing visibility into what's blocking progress.
these unused includes were identified by clang-include-cleaner. after
auditing these source files, all of the reports have been confirmed.
Signed-off-by: Kefu Chai <kefu.chai@scylladb.com>
