[SPARK-7225][SQL] CombineLimits optimizer does not work

SQL
```
select key from (select key from src limit 100) t2 limit 10
```
Optimized Logical Plan before modifying
```
== Optimized Logical Plan ==
Limit 10
Limit 100
Project key#3
MetastoreRelation default, src, None
```
Optimized Logical Plan after modifying
```
== Optimized Logical Plan ==
Limit 10
 Project [key#1]
  MetastoreRelation default, src, None
```

Author: Zhongshuai Pei <799203320@qq.com>
Author: DoingDone9 <799203320@qq.com>

Closes #5770 from DoingDone9/limitOptimizer and squashes the following commits:

c68eaa7 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
97e18cf [Zhongshuai Pei] Update Optimizer.scala
19ab875 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
7db4566 [Zhongshuai Pei] Update CombiningLimitsSuite.scala
e2a491d [Zhongshuai Pei] Update Optimizer.scala
f03fe7f [Zhongshuai Pei] Merge pull request #12 from apache/master
f12fa50 [Zhongshuai Pei] Merge pull request #10 from apache/master
f61210c [Zhongshuai Pei] Merge pull request #9 from apache/master
34b1a9a [Zhongshuai Pei] Merge pull request #8 from apache/master
802261c [DoingDone9] Merge pull request #7 from apache/master
d00303b [DoingDone9] Merge pull request #6 from apache/master
98b134f [DoingDone9] Merge pull request #5 from apache/master
161cae3 [DoingDone9] Merge pull request #4 from apache/master
c87e8b6 [DoingDone9] Merge pull request #3 from apache/master
cb1852d [DoingDone9] Merge pull request #2 from apache/master
c3f046f [DoingDone9] Merge pull request #1 from apache/master