[SPARK-2710] [SQL] Build SchemaRDD from a JdbcRDD with MetaData

SPARK-2710 Build SchemaRDD from a JdbcRDD with MetaData

and a small bug fix on JdbcRDD, line 109
it seems conn will never be closed


here i tried to return a java.sql.ResultSetMetaData object, then build the Seq[(String, Int, Boolean)] for schemaRDD in Spark SQL scope, but when i run this SchemaRDD, i got "java.io.NotSerializableException: org.postgresql.jdbc4.Jdbc4ResultSetMetaData"

so i let this method return a Seq[(String, Int, Boolean)], and in Spark SQL scope, create Seq[StructField] from this Seq[(String, Int, Boolean)]