[SPARK-1092] print warning information if user use SPARK_MEM to regulate executor memory usage

https://spark-project.atlassian.net/browse/SPARK-1092?jql=project%20%3D%20SPARK

print warning information if user set SPARK_MEM to regulate memory usage of executors

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OUTDATED:

Currently, users will usually set SPARK_MEM to control the memory usage of driver programs, (in spark-class)
91 JAVA_OPTS="$OUR_JAVA_OPTS"
92 JAVA_OPTS="$JAVA_OPTS -Djava.library.path=$SPARK_LIBRARY_PATH"
93 JAVA_OPTS="$JAVA_OPTS -Xms$SPARK_MEM -Xmx$SPARK_MEM"
if they didn't set spark.executor.memory, the value in this environment variable will also affect the memory usage of executors, because the following lines in SparkContext
privatespark val executorMemory = conf.getOption("spark.executor.memory")
.orElse(Option(System.getenv("SPARK_MEM")))
.map(Utils.memoryStringToMb)
.getOrElse(512)
also
since SPARK_MEM has been (proposed to) deprecated in SPARK-929 (https://spark-project.atlassian.net/browse/SPARK-929) and the corresponding PR (https://github.com/apache/incubator-spark/pull/104)
we should remove this line

Author: CodingCat <zhunansjtu@gmail.com>

Closes #602 from CodingCat/clean_spark_mem and squashes the following commits:

302bb28 [CodingCat] print warning information if user use SPARK_MEM to regulate executor memory usage
fix for https://spark-project.atlassian.net/browse/SPARK-1052

Author: Bijay Bisht <bijay.bisht@gmail.com>

Closes #568 from bijaybisht/SPARK-1052 and squashes the following commits:

da70395 [Bijay Bisht] fix for https://spark-project.atlassian.net/browse/SPARK-1052 - comments incorporated
fdb1d94 [Bijay Bisht] fix for https://spark-project.atlassian.net/browse/SPARK-1052

(cherry picked from commit e797c1abd9692f1b7ec290e4c83d31fd106e6b05)
Signed-off-by: Aaron Davidson <aaron@databricks.com>
Add subtractByKey to the JavaPairRDD wrapper

Author: Punya Biswal <pbiswal@palantir.com>

Closes #600 from punya/subtractByKey-java and squashes the following commits:

e961913 [Punya Biswal] Hide implicit ClassTags from Java API
c5d317b [Punya Biswal] Add subtractByKey to the JavaPairRDD wrapper
SPARK-1098: Minor cleanup of ClassTag usage in Java API