SPARK-5425: Use synchronised methods in system properties to create SparkConf

SPARK-5425: Fixed usages of system properties

This patch fixes few problems caused by the fact that the Scala wrapper over system properties is not thread-safe and is basically invalid because it doesn't take into account the default values which could have been set in the properties object. The problem is fixed by modifying `Utils.getSystemProperties` method so that it uses `stringPropertyNames` method of the `Properties` class, which is thread-safe (internally it creates a defensive copy in a synchronized method) and returns keys of the properties which were set explicitly and which are defined as defaults.
The other related problem, which is fixed here. was in `ResetSystemProperties` mix-in. It created a copy of the system properties in the wrong way.

This patch also introduces a test case for thread-safeness of SparkConf creation.

Refer to the discussion in https://github.com/apache/spark/pull/4220 for more details.

Author: Jacek Lewandowski <lewandowski.jacek@gmail.com>

Closes #4222 from jacek-lewandowski/SPARK-5425-1.3 and squashes the following commits:

03da61b [Jacek Lewandowski] SPARK-5425: Modified Utils.getSystemProperties to return a map of all system properties - explicit + defaults
8faf2ea [Jacek Lewandowski] SPARK-5425: Use SerializationUtils to save properties in ResetSystemProperties trait
71aa572 [Jacek Lewandowski] SPARK-5425: Use synchronised methods in system properties to create SparkConf

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