As seen in #23284, when the tablet_metadata contains many tables, even empty ones,
we're seeing a long queue of seastar tasks coming from the individual destruction of
`tablet_map_ptr = foreign_ptr<lw_shared_ptr<const tablet_map>>`.
This change improves `tablet_metadata::clear_gently` to destroy the `tablet_map_ptr` objects
on their owner shard by sorting them into vectors, per- owner shard.
Also, background call to clear_gently was added to `~token_metadata`, as it is destroyed
arbitrarily when automatic token_metadata_ptr variables go out of scope, so that the
contained tablet_metadata would be cleared gently.
Finally, a unit test was added to reproduce the `Too long queue accumulated for gossip` symptom
and verify that it is gone with this change.
Fixes#24814
Refs #23284
This change is not marked as fixing the issue since we still need to verify that there is no impact on query performance, reactor stalls, or large allocations, with a large number of tablet-based tables.
* Since the issue exists in 2025.1, requesting backport to 2025.1 and upwards
- (cherry picked from commit 3acca0aa63)
- (cherry picked from commit 493a2303da)
- (cherry picked from commit e0a19b981a)
- (cherry picked from commit 2b2cfaba6e)
- (cherry picked from commit 2c0bafb934)
- (cherry picked from commit 4a3d14a031)
- (cherry picked from commit 6e4803a750)
Parent PR: #24618Closesscylladb/scylladb#24863
* github.com:scylladb/scylladb:
token_metadata_impl: clear_gently: release version tracker early
test: cluster: test_tablets_merge: add test_tablet_split_merge_with_many_tables
token_metadata: clear_and_destroy_impl when destroyed
token_metadata: keep a reference to shared_token_metadata
token_metadata: move make_token_metadata_ptr into shared_token_metadata class
replica: database: get and expose a mutable locator::shared_token_metadata
locator: tablets: tablet_metadata: clear_gently: optimize foreign ptr destruction
fa1b97f0c5