[SPARK-8958] Dynamic allocation: change cached timeout to infinity

pwendell and I discussed this a little more offline and concluded that it would be good to keep it more conservative. Losing cached blocks may be very expensive and we should only allow it if the user knows what he/she is doing.

FYI harishreedharan sryza.

Author: Andrew Or <andrew@databricks.com>

Closes #7329 from andrewor14/da-cached-timeout and squashes the following commits:

cef0b4e [Andrew Or] Change timeout to infinity