Improve concurrent update detection

Caused by: org.nuxeo.ecm.core.storage.StorageException: Could not insert: INSERT INTO "content" ("id", "name", "length", "data", "encoding", "digest", "mime-type") VALUES (?, ?, ?, ?, ?, ?, ?)
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.insertSimpleRows(JDBCRowMapper.java:606)
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.writeCreates(JDBCRowMapper.java:502)
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.write(JDBCRowMapper.java:472)
	at org.nuxeo.ecm.core.storage.sql.SoftRefCachingRowMapper.write(SoftRefCachingRowMapper.java:427)
	at org.nuxeo.ecm.core.storage.sql.SessionImpl.flush(SessionImpl.java:392)
	at org.nuxeo.ecm.core.storage.sql.SessionImpl.flushAndScheduleWork(SessionImpl.java:366)
	at org.nuxeo.ecm.core.storage.sql.SessionImpl.end(SessionImpl.java:1457)
	... 17 more
Caused by: java.sql.BatchUpdateException: Batch entry 0 INSERT INTO "content" ("id", "name", "length", "data", "encoding", "digest", "mime-type") VALUES ('8499121f-7b68-4558-bd7e-cfb0fa485729', 'b21b52769a8839d31a50ee58349f6df8', 423, 'b21b52769a8839d31a50ee58349f6df8', NULL, NULL, 'application/octet-stream') was aborted.  Call getNextException to see the cause.
	at org.postgresql.jdbc2.AbstractJdbc2Statement$BatchResultHandler.handleError(AbstractJdbc2Statement.java:2746)
	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1887)
	at org.postgresql.core.v3.QueryExecutorImpl.execute(QueryExecutorImpl.java:405)
	at org.postgresql.jdbc2.AbstractJdbc2Statement.executeBatch(AbstractJdbc2Statement.java:2893)
	at org.apache.tomcat.dbcp.dbcp.DelegatingStatement.executeBatch(DelegatingStatement.java:297)
	at org.apache.tomcat.dbcp.dbcp.DelegatingStatement.executeBatch(DelegatingStatement.java:297)
	at org.nuxeo.ecm.core.storage.sql.jdbc.JDBCRowMapper.insertSimpleRows(JDBCRowMapper.java:590)
	... 23 more
Caused by: org.postgresql.util.PSQLException: ERROR: duplicate key value violates unique constraint "content_pk"
  Detail: Key (id)=(8499121f-7b68-4558-bd7e-cfb0fa485729) already exists.
	at org.postgresql.core.v3.QueryExecutorImpl.receiveErrorResponse(QueryExecutorImpl.java:2157)
	at org.postgresql.core.v3.QueryExecutorImpl.processResults(QueryExecutorImpl.java:1886)
	... 28 more