[CALCITE-1527] Support DML in the JDBC adapter

[tzolov]

• I had to align the SqlUpdate's RowType with the rest of DML operations [42a5461]. Note that the DML operations (update including) return rowcount instead of ResultSet
• UPDATE implementation requires access to the SET source expressions. While TableModify provides the SET column names it misses the set value expressions. Therefore I've added a new field: List<RexNode> sourceExpressionList to TableModify (and updated many impacted classes ) [9c8f141].
  The sourceExpressionList is initialized from the SqlUpdate:

private RelNode convertUpdate(SqlUpdate call) {
    final SqlValidatorScope scope = validator.getWhereScope(call.getSourceSelect());
    Blackboard bb = createBlackboard(scope, null, false);
    Builder<RexNode> rexNodeSourceExpressionListBuilder = ImmutableList.builder();
    for (SqlNode n: call.getSourceExpressionList()) {
      RexNode rn = bb.convertExpression(n);
      rexNodeSourceExpressionListBuilder.add(rn);
    }
....

It works for the common use cases but failed with update sub-queries. Check the RelToSqlConverter#testUpdateSubQuery! I guess there is a better way to resolve the source expressions for sub-query?

• [658cc90] Implements the Rel to SQL for the INSERT, UPDATE and DELETE operations. Note that DML returns row count instead of ResultSet. To support DML result i had to change the hardcoded statement.executeQuery(sql) (in ResultSetEnumerable) into boolean isResultSetProvided = statement.execute(sql) and handle the result according to the isResultSetProvided.
  To Support the Insert i had to make the JdbcTable a ModifiableTable - later is marked as experimental?
  Also i had to lower the JdbcTableModify cost (super.computeSelfCost(planner, mq).multiplyBy(.1)) to make it win over the EnumberableTableModify.

[tzolov]

Note: [edc28ad] resolves a strange problem that i unintentionally have caused. Apparently we need to keep the connection and statement open after the execute(sql). By setting their fields to null we prevent the final block from closing them.