Before waiting on stale_versions_in_use(), we log the stale versions
the barrier_and_drain handler will wait for, along with the number of
token_metadata references representing each version.
To achieve this, we store a pointer to token_metadata in
version_tracker, traverse the _trackers list, and output all items
with a version smaller than the latest. Since token_metadata
contains the version_tracker instance, it is guaranteed to remain
alive during traversal. To count references, token_metadata now
inherits from enable_lw_shared_from_this.
This helps diagnose tablet migration stalls and allows more
deterministic tests: when a barrier is expected to block, we can
verify that the log contains the expected stale versions rather
than checking that the barrier_and_drain is blocked on
stale_versions_in_use() for a fixed amount of time.
e39f4b399c