safekeeper: check for non-consecutive writes in safekeeper.rs

wal_storage.rs already checks this, but since this is a quite legit scenario
check it at safekeeper.rs (consensus level) as well.

ref https://github.com/neondatabase/neon/issues/8212

This is a take 2; previous PR #8640 had been reverted because interplay
with another change broke test_last_log_term_switch.

safekeeper/tests/walproposer_sim/safekeeper_disk.rs

@@ -175,6 +175,10 @@ impl DiskWALStorage {
 }
 
 impl wal_storage::Storage for DiskWALStorage {
+    // Last written LSN.
+    fn write_lsn(&self) -> Lsn {
+        self.write_lsn
+    }
     /// LSN of last durably stored WAL record.
     fn flush_lsn(&self) -> Lsn {
         self.flush_record_lsn