[SPARK-3938][SQL] Names in-memory columnar RDD with corresponding table name

This PR enables the Web UI storage tab to show the in-memory table name instead of the mysterious query plan string as the name of the in-memory columnar RDD.

Note that after #2501, a single columnar RDD can be shared by multiple in-memory tables, as long as their query results are the same. In this case, only the first cached table name is shown. For example:

```sql
CACHE TABLE first AS SELECT * FROM src;
CACHE TABLE second AS SELECT * FROM src;
```

The Web UI only shows "In-memory table first".

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Author: Cheng Lian <lian@databricks.com>

Closes #3383 from liancheng/columnar-rdd-name and squashes the following commits:

071907f [Cheng Lian] Fixes tests
12ddfa6 [Cheng Lian] Names in-memory columnar RDD with corresponding table name
[SPARK-4228][SQL] SchemaRDD to JSON

Here's a simple fix for SchemaRDD to JSON.

Author: Dan McClary <dan.mcclary@gmail.com>

Closes #3213 from dwmclary/SPARK-4228 and squashes the following commits:

d714e1d [Dan McClary] fixed PEP 8 error
cac2879 [Dan McClary] move pyspark comment and doctest to correct location
f9471d3 [Dan McClary] added pyspark doc and doctest
6598cee [Dan McClary] adding complex type queries
1a5fd30 [Dan McClary] removing SPARK-4228 from SQLQuerySuite
4a651f0 [Dan McClary] cleaned PEP and Scala style failures.  Moved tests to JsonSuite
47ceff6 [Dan McClary] cleaned up scala style issues
2ee1e70 [Dan McClary] moved rowToJSON to JsonRDD
4387dd5 [Dan McClary] Added UserDefinedType, cleaned up case formatting
8f7bfb6 [Dan McClary] Map type added to SchemaRDD.toJSON
1b11980 [Dan McClary] Map and UserDefinedTypes partially done
11d2016 [Dan McClary] formatting and unicode deserialization default fixed
6af72d1 [Dan McClary] deleted extaneous comment
4d11c0c [Dan McClary] JsonFactory rewrite of toJSON for SchemaRDD
149dafd [Dan McClary] wrapped scala toJSON in sql.py
5e5eb1b [Dan McClary] switched to Jackson for JSON processing
6c94a54 [Dan McClary] added toJSON to pyspark SchemaRDD
aaeba58 [Dan McClary] added toJSON to pyspark SchemaRDD
1d171aa [Dan McClary] upated missing brace on if statement
319e3ba [Dan McClary] updated to upstream master with merged SPARK-4228
424f130 [Dan McClary] tests pass, ready for pull and PR
626a5b1 [Dan McClary] added toJSON to SchemaRDD
f7d166a [Dan McClary] added toJSON method
5d34e37 [Dan McClary] merge resolved
d6d19e9 [Dan McClary] pr example