1. All nodes in the cluster have to support MC_SSTABLE_FEATURE
2. When a node observes that whole cluster supports MC_SSTABLE_FEATURE
then it should start using MC format.
3. Once all shards start to use MC then a node should broadcast that
unbounded range tombstones are now supported by the cluster.
4. Once whole cluster supports unbounded range tombstones we can
start accepting them on CQL level.
tests: unit(release)
Fixes#4205Fixes#4113
* seastar-dev.git dev/haaawk/enable_mc/v11:
system_keyspace: Add scylla_local
system_keyspace: add accessors for SCYLLA_LOCAL
storage_service: add _sstables_format field
feature: add when_enabled callbacks
system_keyspace: add storage_service param to setup
Add sstable format helper methods
Register feature listeners in storage_service
Add service::read_sstables_format
Use read_sstables_format in main.cc
Use _sstables_format to determine current format
Add _unbounded_range_tombstones_feature
Update supported features on format change