ZEPPELIN-487 Change supporting multiple statements to multiple connections

[jongyoul]

Changed multiple statements to multiple connection. Some JDBC don't support parallel executions with one connection.

[prasadwagle]

I tested the pull request and it allowed Vertica queries to execute in parallel with one modification. Thanks!

I had to comment:
// if (statement.isClosed()) {
// connection = getConnection(propertyKey);
// statement = connection.createStatement();
// }
since I ran into:
java.lang.AbstractMethodError: com.vertica.jdbc.VerticaStatementImpl.isClosed()Z
at org.apache.zeppelin.hive.HiveInterpreter.getStatement(HiveInterpreter.java:208)

Can you help me understand the need for the statement.isClosed() check in line 208 since the statement is created in line 207?

[jongyoul]

It's soft of defensive code for avoiding error. Do you think it's redundant? I'll fix this code more general :-). Thanks for the review quickly.

[jongyoul]

@praagarw Could you please review this PR again?

[prasadwagle]

Hi Jongyoul, since createStatement is implemented in the JDBC drivers, I think it is unlikely it will return a closed statement. However, I am all for defensive coding and your fix in ZEPPELIN-487 looks good to me.

[jongyoul]

@praagarw Sure, but I couldn't guarantee all of jdbc connections create statement when it's alive. Thus double check might be helpful in that case. And thanks for kind review. I'm merging in three day.

[jongyoul]

Merging without additional discussion