bug fixes in both backends. add ledger_range rpc. improve test scripts

handlers/LedgerRange.cpp

@@ -0,0 +1,22 @@
+#include <handlers/RPCHelpers.h>
+#include <reporting/BackendInterface.h>
+
+boost::json::object
+doLedgerRange(
+    boost::json::object const& request,
+    BackendInterface const& backend)
+{
+    boost::json::object response;
+
+    auto range = backend.fetchLedgerRange();
+    if (!range)
+    {
+        response["error"] = "No data";
+    }
+    else
+    {
+        response["ledger_index_min"] = range->minSequence;
+        response["ledger_index_max"] = range->maxSequence;
+    }
+    return response;
+}