DPL Analysis: query the CCDB once per run of equal timestamps

When a CCDB table has no separate uniformity column, the analysis CCDB
fetcher called populateCacheWith for every row of the source table, so
about 19k times per timeframe for a BC-indexed table such as the ones of
propagation-service-v2, even though consecutive BCs share the same
millisecond timestamp. Reuse the previous responses whenever the lookup
key, the uniformity value or else the timestamp, is the same as the one
of the previous row. Since the key determines every FetchOp of the row,
the responses and hence the output table are unchanged.

For the LHC25b4a6 AO2D of Hyperloop test 799098 this is about 350 runs of
equal (millisecond) BC times over 1.17M BCs in 61 DFs, instead of one
query per BC.

Author: ktf

Framework/CCDBSupport/src/AnalysisCCDBHelpers.cxx

@@ -235,8 +235,8 @@ AlgorithmSpec AnalysisCCDBHelpers::fetchFromCCDB(ConfigContext const& /*ctx*/)
 
         // Rows sharing a uniformity value resolve to the same objects, so the query is
         // issued once per distinct value and the resulting handles are repeated for the
-        // rest of the run. When uniformity is the timestamp itself (the default) this
-        // degenerates to the previous behaviour, one query per row.
+        // rest of the run. When uniformity is the timestamp itself (the default) the
+        // query is issued once per run of consecutive rows with the same timestamp.
         std::vector<int64_t> uniformity;
         bool const shortCircuit = uniformityColumn.get() != timestampColumn.get();
         if (shortCircuit) {
@@ -273,15 +273,13 @@ AlgorithmSpec AnalysisCCDBHelpers::fetchFromCCDB(ConfigContext const& /*ctx*/)
 
           for (int64_t ri = 0; ri < chunk->data()->length; ri++) {
             ++row;
-            bool const sameAsPrevious = shortCircuit && haveResponses && uniformity[row] == previousUniformity;
-            if (shortCircuit) {
-              previousUniformity = uniformity[row];
-            }
             ops.clear();
             int64_t timestamp = timestamps[ri];
             // Key the path lookup on the uniformity value; when uniformity is the
             // timestamp itself the mapping expresses validity intervals instead.
             int64_t const uniformityKey = shortCircuit ? uniformity[row] : timestamp;
+            bool const sameAsPrevious = haveResponses && uniformityKey == previousUniformity;
+            previousUniformity = uniformityKey;
             int fi = 0;
             for (auto& field : schema->fields()) {
               auto const& url = pathTables[i][fi++].resolve(uniformityKey, field->name());