[SPARK-5614][SQL] Predicate pushdown through Generate.

Now in Catalyst's rules, predicates can not be pushed through "Generate" nodes. Further more, partition pruning in HiveTableScan can not be applied on those queries involves "Generate". This makes such queries very inefficient. In practice, it finds patterns like

```scala
Filter(predicate, Generate(generator, _, _, _, grandChild))
```

and splits the predicate into 2 parts by referencing the generated column from Generate node or not. And a new Filter will be created for those conjuncts can be pushed beneath Generate node. If nothing left for the original Filter, it will be removed.
For example, physical plan for query
```sql
select len, bk
from s_server lateral view explode(len_arr) len_table as len
where len > 5 and day = '20150102';
```
where 'day' is a partition column in metastore is like this in current version of Spark SQL:

> Project [len, bk]
>
> Filter ((len > "5") && "(day = "20150102")")
>
> Generate explode(len_arr), true, false
>
> HiveTableScan [bk, len_arr, day], (MetastoreRelation default, s_server, None), None

But theoretically the plan should be like this

> Project [len, bk]
>
> Filter (len > "5")
>
> Generate explode(len_arr), true, false
>
> HiveTableScan [bk, len_arr, day], (MetastoreRelation default, s_server, None), Some(day = "20150102")

Where partition pruning predicates can be pushed to HiveTableScan nodes.

Author: Lu Yan <luyan02@baidu.com>

Closes #4394 from ianluyan/ppd and squashes the following commits:

a67dce9 [Lu Yan] Fix English grammar.
7cea911 [Lu Yan] Revised based on @marmbrus's opinions
ffc59fc [Lu Yan] [SPARK-5614][SQL] Predicate pushdown through Generate.