3f7ee3ce5d introduced system.batchlog_v2, with a schema designed to speed up batchlog replays and make post-replay cleanups much more effective.
It did not introduce a cluster feature for the new table, because it is node local table, so the cluster can switch to the new table gradually, one node at a time.
However, https://github.com/scylladb/scylladb/issues/27886 showed that the switching causes timeouts during upgrades, in mixed clusters. Furthermore, switching to the new table unconditionally on upgrades nodes, means that on rollback, the batches saved into the v2 table are lost.
This PR introduces re-introduces v1 (`system.batchlog`) support and guards the use of the v2 table with a cluster feature, so mixed clusters keep using v1 and thus be rollback-compatible.
The re-introduced v1 support doesn't support post-replay cleanups for simplicity. The cleanup in v1 was never particularly effective anyway and we ended up disabling it for heavy batchlog users, so I don't think the lack of support for cleanup is a problem.
Fixes: https://github.com/scylladb/scylladb/issues/27886
Needs backport to 2026.1, to fix upgrades for clusters using batches
Closesscylladb/scylladb#28736
* github.com:scylladb/scylladb:
test/boost/batchlog_manager_test: add tests for v1 batchlog
test/boost/batchlog_manager_test: make prepare_batches() work with both v1 and v2
test/boost/batchlog_manager_test: fix indentation
test/boost/batchlog_manager_test: extract prepare_batches() method
test/lib/cql_assertions: is_rows(): add dump parameter
tools/scylla-sstable: extract query result printers
tools/scylla-sstable: add std::ostream& arg to query result printers
repair/row_level: repair_flush_hints_batchlog_handler(): add all_replayed to finish log
db/batchlog_manager: re-add v1 support
db/batchlog_manager: return all_replayed from process_batch()
db/batchlog_manager: process_bath() fix indentation
db/batchlog_manager: make batch() a standalone function
db/batchlog_manager: make structs stats public
db/batchlog_manager: allocate limiter on the stack
db/batchlog_manager: add feature_service dependency
gms/feature_service: add batchlog_v2 feature
a83ee6cf66