[SPARK-5073] spark.storage.memoryMapThreshold have two default value

Because major OS page sizes is about 4KB, the default value of spark.storage.memoryMapThreshold is integrated to 2 * 4096

Author: lewuathe <lewuathe@me.com>

Closes #3900 from Lewuathe/integrate-memoryMapThreshold and squashes the following commits:

e417acd [lewuathe] [SPARK-5073] Update docs/configuration
834aba4 [lewuathe] [SPARK-5073] Fix style
adcea33 [lewuathe] [SPARK-5073] Integrate memory map threshold to 2MB
fcce2e5 [lewuathe] [SPARK-5073] spark.storage.memoryMapThreshold have two default value