[SPARK-1669][SQL]  Made cacheTable idempotent

JIRA issue: [SPARK-1669](https://issues.apache.org/jira/browse/SPARK-1669)

Caching the same table multiple times should end up with only 1 in-memory columnar representation of this table.

Before:

```
scala> loadTestTable("src")
...
scala> cacheTable("src")
...
scala> cacheTable("src")
...
scala> table("src")
...
== Query Plan ==
InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (HiveTableScan [key#2,value#3], (MetastoreRelation default, src, None), None))))
```

After:

```
scala> loadTestTable("src")
...
scala> cacheTable("src")
...
scala> cacheTable("src")
...
scala> table("src")
...
== Query Plan ==
InMemoryColumnarTableScan [key#2,value#3], (InMemoryRelation [key#2,value#3], false, (HiveTableScan [key#2,value#3], (MetastoreRelation default, src, None), None))
```

Author: Cheng Lian <lian.cs.zju@gmail.com>

Closes #1183 from liancheng/spark-1669 and squashes the following commits:

68f8a20 [Cheng Lian] Removed an unused import
51bae90 [Cheng Lian] Made cacheTable idempotent