raft: server: track last persisted snapshot descriptor index

Also introduce a condition variable notified whenever this index is
updated.

Will be user in following commits.

raft/server.cc

@@ -122,6 +122,8 @@ private:
     std::optional<shared_promise<>> _state_change_promise;
     // Index of the last entry applied to `_state_machine`.
     index_t _applied_idx;
+    // Index of the last persisted snapshot descriptor.
+    index_t _snapshot_desc_idx;
     std::list<active_read> _reads;
     std::multimap<index_t, awaited_index> _awaited_indexes;
 
@@ -134,6 +136,9 @@ private:
     // Signaled when apply index is changed
     condition_variable _applied_index_changed;
 
+    // Signaled when _snapshot_desc_idx is changed
+    condition_variable _snapshot_desc_idx_changed;
+
     struct stop_apply_fiber{}; // exception to send when apply fiber is needs to be stopepd
 
     struct removed_from_config{}; // sent to applier_fiber when we're not a leader and we're outside the current configuration
@@ -371,9 +376,10 @@ future<> server_impl::start() {
                                  _events);
 
     _applied_idx = index_t{0};
+    _snapshot_desc_idx = index_t{0};
     if (snapshot.id) {
         co_await _state_machine->load_snapshot(snapshot.id);
-        _applied_idx = snapshot.idx;
+        _snapshot_desc_idx = _applied_idx = snapshot.idx;
     }
 
     if (!rpc_config.current.empty()) {
@@ -1017,6 +1023,8 @@ future<> server_impl::process_fsm_output(index_t& last_stable, fsm_output&& batc
         logger.trace("[{}] io_fiber storing snapshot {}", _id, snp.id);
         // Persist the snapshot
         co_await _persistence->store_snapshot_descriptor(snp, max_trailing_entries);
+        _snapshot_desc_idx = snp.idx;
+        _snapshot_desc_idx_changed.broadcast();
         _stats.store_snapshot++;
         // If this is locally generated snapshot there is no need to
         // load it.