Issue #747: fix flaky MS SQL MERGE upsert PRIMARY KEY violation with WITH (HOLDLOCK)

The JDBC backend performs the SQL Server upsert via MERGE ... WHEN NOT MATCHED
THEN INSERT. Under READ_COMMITTED, SQL Server's MERGE is not atomic: two
concurrent sessions can both evaluate NOT MATCHED for the same key and both
attempt the INSERT, producing a duplicate PRIMARY KEY error. This is the race
that intermittently fails PluggableBackendImplTestCase.test_issue_496_2 (8
threads upserting the same key).

Add the WITH (HOLDLOCK) (SERIALIZABLE range-lock) table hint to the MERGE
target so the existence check and the insert are atomic with respect to other
sessions, serializing concurrent upserts of the same key.

Author: vharseko

opendj-server-legacy/src/main/java/org/opends/server/backends/jdbc/Storage.java

@@ -389,8 +389,8 @@ boolean upsert(TreeName treeName, ByteSequence key, ByteSequence value) throws S
 					statement.setBytes(3, value.toByteArray());
 					return (execute(statement) == 1 && statement.getUpdateCount() > 0);
 				}
-			}else if (driverName.contains("microsoft")) { //ANSI MERGE with ;
-				try (final PreparedStatement statement = con.prepareStatement("merge into " + getTableName(treeName) + " old using (select ? h,? k,? v) new on (old.h=new.h and old.k=new.k) WHEN MATCHED THEN UPDATE SET old.v=new.v WHEN NOT MATCHED THEN INSERT (h,k,v) VALUES (new.h,new.k,new.v);")) {
+			}else if (driverName.contains("microsoft")) { //ANSI MERGE with ; WITH (HOLDLOCK) makes the upsert atomic: without it SQL Server MERGE can race two concurrent NOT MATCHED inserts of the same key into a PRIMARY KEY violation
+				try (final PreparedStatement statement = con.prepareStatement("merge into " + getTableName(treeName) + " WITH (HOLDLOCK) old using (select ? h,? k,? v) new on (old.h=new.h and old.k=new.k) WHEN MATCHED THEN UPDATE SET old.v=new.v WHEN NOT MATCHED THEN INSERT (h,k,v) VALUES (new.h,new.k,new.v);")) {
 					statement.setString(1, key2hash.get(ByteBuffer.wrap(key.toByteArray())));
 					statement.setBytes(2, real2db(key.toByteArray()));
 					statement.setBytes(3, value.toByteArray());