[SPARK-6024][SQL] When a data source table has too many columns, it's schema...

[SPARK-6024][SQL] When a data source table has too many columns, it's schema cannot be stored in metastore.

JIRA: https://issues.apache.org/jira/browse/SPARK-6024

Author: Yin Huai <yhuai@databricks.com>

Closes #4795 from yhuai/wideSchema and squashes the following commits:

4882e6f [Yin Huai] Address comments.
73e71b4 [Yin Huai] Address comments.
143927a [Yin Huai] Simplify code.
cc1d472 [Yin Huai] Make the schema wider.
12bacae [Yin Huai] If the JSON string of a schema is too large, split it before storing it in metastore.
e9b4f70 [Yin Huai] Failed test.