Fix to call isValid and Adds appender increment control

[caiooliveiraeti]

1 - Fix to call isValid connection only if necessary If call isValid in Ms Sql Server and has ResultSet is getting data in background jdbc driver has a problem. This solve a issue #361. Call isValid only if need make a query. 2 - Adds appender increment control

[ghost]

@confluentinc It looks like @caiooliveiraeti just signed our Contributor License Agreement. :+1:

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clabot

[rhauch]

This likely needs to take into account the changes from #333. I believe that we've already addressed the "isValid" connection problem, since the table querier was already changed to take a cached connection provider.

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