tablets: Implement cleanup step

This change adds a stub for tablet cleanup on the replica side and wires
it into the tablet migration process.

The handling on replica side is incomplete because it doesn't remove
the actual data yet. It only flushes the memtables, so that all data
is in sstables and none requires a memtable flush.

This patch is necessary to make decommission work. Otherwise, a
memtable flush would happen when the decommissioned node is put in the
drained state (as in nodetool drain) and it would fail on missing host
id mapping (node is no longer in topology), which is examined by the
tablet sharder when producing sstable sharding metadata. Leading to
abort due to failed memtable flush.

locator/tablets.cc

@@ -45,6 +45,8 @@ write_replica_set_selector get_selector_for_writes(tablet_transition_stage stage
             return write_replica_set_selector::next;
         case tablet_transition_stage::cleanup:
             return write_replica_set_selector::next;
+        case tablet_transition_stage::end_migration:
+            return write_replica_set_selector::next;
     }
     on_internal_error(tablet_logger, format("Invalid tablet transition stage: {}", static_cast<int>(stage)));
 }
@@ -64,6 +66,8 @@ read_replica_set_selector get_selector_for_reads(tablet_transition_stage stage)
             return read_replica_set_selector::next;
         case tablet_transition_stage::cleanup:
             return read_replica_set_selector::next;
+        case tablet_transition_stage::end_migration:
+            return read_replica_set_selector::next;
     }
     on_internal_error(tablet_logger, format("Invalid tablet transition stage: {}", static_cast<int>(stage)));
 }
@@ -232,6 +236,7 @@ static const std::unordered_map<tablet_transition_stage, sstring> tablet_transit
     {tablet_transition_stage::streaming, "streaming"},
     {tablet_transition_stage::use_new, "use_new"},
     {tablet_transition_stage::cleanup, "cleanup"},
+    {tablet_transition_stage::end_migration, "end_migration"},
 };
 
 static const std::unordered_map<sstring, tablet_transition_stage> tablet_transition_stage_from_name = std::invoke([] {

locator/tablets.hh

@@ -141,6 +141,7 @@ enum class tablet_transition_stage {
     write_both_read_new,
     use_new,
     cleanup,
+    end_migration,
 };
 
 sstring tablet_transition_stage_to_string(tablet_transition_stage);