[SPARK-7897] Improbe type for jdbc/"unsigned bigint"

The original fix uses DecimalType.Unlimited, which is harder to
handle afterwards. There is no scale and most data should fit into
a long, thus DecimalType(20,0) should be better.

Author: Rene Treffer <treffer@measite.de>

Closes apache#6789 from rtreffer/spark-7897-unsigned-bigint-as-decimal and squashes the following commits:

2006613 [Rene Treffer] Fix type for "unsigned bigint" jdbc loading.

sql/core/src/main/scala/org/apache/spark/sql/jdbc/JDBCRDD.scala

@@ -55,7 +55,7 @@ private[sql] object JDBCRDD extends Logging {
     val answer = sqlType match {
       // scalastyle:off
       case java.sql.Types.ARRAY         => null
-      case java.sql.Types.BIGINT        => if (signed) { LongType } else { DecimalType.Unlimited }
+      case java.sql.Types.BIGINT        => if (signed) { LongType } else { DecimalType(20,0) }
       case java.sql.Types.BINARY        => BinaryType
       case java.sql.Types.BIT           => BooleanType // @see JdbcDialect for quirks
       case java.sql.Types.BLOB          => BinaryType