sprint() recently became more strict, throwing on sprint("%s", 5). Replace
with the more modern format().
Mechanically converted with https://github.com/avikivity/unsprint.
Before this patch, maybe_merge_versions() had to be manually called
before partition snapshot goes away. That is error prone and makes
client code more complicated. Delegate that task to a new
partition_snapshot_ptr object, through which all snapshots are
published now.
Now all snapshots will have a mutation_cleaner which they will use to
gently destroy freed partition_version objects.
Destruction of memtable entries during cache update is also using the
gentle cleaner now. We need to have a separate cleaner for memtable
objects even though they're owned by cache's region, because memtable
versions must be cleared without a cache_tracker.
Each memtable will have its own cleaner, which will be merged with the
cache's cleaner when memtable is merged into cache.
Fixes some sources of reactor stalls on cache update when there are
large partition entries in memtables.
