CNBD-12239: Replace System.nanoTime with approxTime in SAI where appropriate (#1469)

Fixes riptano/cndb#12239

We found the System.nanoTime was using significant cpu cost, but because
the timeout is high enough, we can accept the inaccuracy.

- [ ] Make sure there is a PR in the CNDB project updating the Converged
Cassandra version
- [ ] Use `NoSpamLogger` for log lines that may appear frequently in the
logs
- [ ] Verify test results on Butler
- [ ] Test coverage for new/modified code is > 80%
- [ ] Proper code formatting
- [ ] Proper title for each commit staring with the project-issue
number, like CNDB-1234
- [ ] Each commit has a meaningful description
- [ ] Each commit is not very long and contains related changes
- [ ] Renames, moves and reformatting are in distinct commits

Author: michaeljmarshall

src/java/org/apache/cassandra/index/sai/QueryContext.java

@@ -27,8 +27,8 @@
 import org.apache.cassandra.config.CassandraRelevantProperties;
 import org.apache.cassandra.config.DatabaseDescriptor;
 import org.apache.cassandra.index.sai.utils.AbortedOperationException;
+import org.apache.cassandra.utils.MonotonicClock;
 
-import static org.apache.cassandra.utils.Clock.Global.nanoTime;
 import static java.lang.Math.max;
 
 /**
@@ -79,12 +79,12 @@ public QueryContext()
     public QueryContext(long executionQuotaMs)
     {
         this.executionQuotaNano = TimeUnit.MILLISECONDS.toNanos(executionQuotaMs);
-        queryStartTimeNanos = nanoTime();
+        this.queryStartTimeNanos = MonotonicClock.Global.approxTime.now();
     }
 
     public long totalQueryTimeNs()
     {
-        return nanoTime() - queryStartTimeNanos;
+        return MonotonicClock.Global.approxTime.now() - queryStartTimeNanos;
     }
 
     // setters

src/java/org/apache/cassandra/index/sai/plan/QueryView.java

@@ -43,7 +43,7 @@
 import org.apache.cassandra.io.sstable.format.SSTableReader;
 import org.apache.cassandra.io.sstable.format.SSTableReaderWithFilter;
 import org.apache.cassandra.tracing.Tracing;
-import org.apache.cassandra.utils.Clock;
+import org.apache.cassandra.utils.MonotonicClock;
 import org.apache.cassandra.utils.NoSpamLogger;
 
 public class QueryView implements AutoCloseable
@@ -162,11 +162,11 @@ protected QueryView build()
                             // Log about the failures
                             if (failingSince <= 0)
                             {
-                                failingSince = Clock.Global.nanoTime();
+                                failingSince = MonotonicClock.Global.approxTime.now();
                             }
-                            else if (Clock.Global.nanoTime() - failingSince > TimeUnit.MILLISECONDS.toNanos(100))
+                            else if (MonotonicClock.Global.approxTime.now() - failingSince > TimeUnit.MILLISECONDS.toNanos(100))
                             {
-                                failingSince = Clock.Global.nanoTime();
+                                failingSince = MonotonicClock.Global.approxTime.now();
                                 if (success)
                                     NoSpamLogger.log(logger, NoSpamLogger.Level.WARN, 1, TimeUnit.SECONDS,
                                                      "Spinning trying to capture index reader for {}, but it was released.", index);