ETL fixes

* Don't use cache if cache is more recent than extracted ledger
* Use hardFetchLedgerRangeNoThrow exclusively in ETL, to prevent race
  condition

src/backend/SimpleCache.cpp

@@ -1,5 +1,11 @@
 #include <backend/SimpleCache.h>
 namespace Backend {
+uint32_t
+SimpleCache::latestLedgerSequence()
+{
+    std::shared_lock lck{mtx_};
+    return latestSeq_;
+}
 
 void
 SimpleCache::update(