Fixes#14870
(Originally suggested by @avikivity). Use commit log stored GC clock min positions to narrow compaction GC bounds.
(Still requires augmented manual flush:es with extensive CL clearing to pass various dtest, but this does not affect "real" execution).
Adds a lowest timestamp of GC clock whenever a CF is added to a CL segment the first time. Because GC clock is wall
clock time and only connected to TTL (not cell/row timestamps), this gives a fairly accurate view of GC low bounds
per segment. This is then (in a rather ugly way) propagated to tombstone_gc_state to narrow the allowed GC bounds for
a CF, based on what is currently left in CL.
Note: this is a rather unoptimized version - no caching or anything. But even so, should not be excessively expensive,
esp. since various other code paths already cache the results.
Closesscylladb/scylladb#15060
* github.com:scylladb/scylladb:
main/cql_test_env: Augment compaction mgr tombstone_gc_state with CL GC info
tombstone_gc_state: Add optional callback to augment GC bounds
commitlog: Add keeping track of approximate lowest GC clock for CF entries
database: Force new commitlog segment on user initiated flush
commitlog: Add helper to force new active segment
f42eb4d1ce