[SPARK-20337][CORE] Support upgrade a jar dependency and don't restart SparkContext

[wangyum]

What changes were proposed in this pull request?

Support upgrade a jar dependency and don't restart SparkContext, Something like this:

sc.addJar("breeze-natives_2.11-0.12.jar")
// do something
sc.removeJar("spark://192.168.26.200:23420/jar/breeze-natives_2.11-0.12.jar")
sc.addJar("breeze-natives_2.11-0.13.jar")
// do something

How was this patch tested?

unit tests and manual tests

[SparkQA]

Test build #75806 has finished for PR 17637 at commit eb4cb86.

• This patch passes all tests.
• This patch merges cleanly.
• This patch adds no public classes.

[vanzin]

This does not work. Any classes that have already been loaded from the old jar will not be unloaded. So you're going to end up with really odd issues when two classes from different jars don't agree with each other.