SPARK-1999: StorageLevel in storage tab and RDD Storage Info never changes

StorageLevel in 'storage tab' and 'RDD Storage Info' never changes even if you call rdd.unpersist() and then you give the rdd another different storage level.

Author: CrazyJvm <crazyjvm@gmail.com>

Closes #968 from CrazyJvm/ui-storagelevel and squashes the following commits:

62555fa [CrazyJvm] change RDDInfo constructor param 'storageLevel' to var, so there's need to add another variable _storageLevel。
9f1571e [CrazyJvm] JIRA https://issues.apache.org/jira/browse/SPARK-1999 UI : StorageLevel in storage tab and RDD Storage Info never changes
SPARK-2148 Add link to requirements for custom equals() and hashcode() methods

https://issues.apache.org/jira/browse/SPARK-2148

Author: Andrew Ash <andrew@andrewash.com>

Closes #1092 from ash211/SPARK-2148 and squashes the following commits:

93513df [Andrew Ash] SPARK-2148 Add link to requirements for custom equals() and hashcode() methods
(cherry picked from commit 9672ee07fb1c3583c70f23a699de3b2282eb0f98)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>
Updating docs to include missing information about reducers and clarify ...

...how the OFFHEAP storage level works (there has been confusion around this).

Author: Ali Ghodsi <alig@cs.berkeley.edu>

Closes #1089 from alig/master and squashes the following commits:

ca8114d [Ali Ghodsi] Updating docs to include missing information about reducers and clarify how the OFFHEAP storage level works (there has been confusion around this).
(cherry picked from commit 119b06a04f6df3949b3b074a18f791bbc732ac31)

Signed-off-by: Patrick Wendell <pwendell@gmail.com>