Remove extra semicolon in Task.scala

Author: GuoQiang Li <witgo@qq.com>

Closes #1876 from witgo/remove_semicolon_in_Task_scala and squashes the following commits:

c6ea732 [GuoQiang Li] Remove extra semicolon in Task.scala

(cherry picked from commit 3570119c34ab8d61507e7703a171b742fb0957d4)
Signed-off-by: Reynold Xin <rxin@apache.org>
[SPARK-2898] [PySpark] fix bugs in deamon.py

1. do not use signal handler for SIGCHILD, it's easy to cause deadlock
2. handle EINTR during accept()
3. pass errno into JVM
4. handle EAGAIN during fork()

Now, it can pass 50k tasks tests in 180 seconds.

Author: Davies Liu <davies.liu@gmail.com>

Closes #1842 from davies/qa and squashes the following commits:

f0ea451 [Davies Liu] fix lint
03a2e8c [Davies Liu] cleanup dead children every seconds
32cb829 [Davies Liu] fix lint
0cd0817 [Davies Liu] fix bugs in deamon.py

(cherry picked from commit 28dcbb531ae57dc50f15ad9df6c31022731669c9)
Signed-off-by: Josh Rosen <joshrosen@apache.org>
[SPARK-2937] Separate out samplyByKeyExact as its own API in PairRDDFunction

To enable Python consistency and `Experimental` label of the `sampleByKeyExact` API.

Author: Doris Xin <doris.s.xin@gmail.com>
Author: Xiangrui Meng <meng@databricks.com>

Closes #1866 from dorx/stratified and squashes the following commits:

0ad97b2 [Doris Xin] reviewer comments.
2948aae [Doris Xin] remove unrelated changes
e990325 [Doris Xin] Merge branch 'master' into stratified
555a3f9 [Doris Xin] separate out sampleByKeyExact as its own API
616e55c [Doris Xin] merge master
245439e [Doris Xin] moved minSamplingRate to getUpperBound
eaf5771 [Doris Xin] bug fixes.
17a381b [Doris Xin] fixed a merge issue and a failed unit
ea7d27f [Doris Xin] merge master
b223529 [Xiangrui Meng] use approx bounds for poisson fix poisson mean for waitlisting add unit tests for Java
b3013a4 [Xiangrui Meng] move math3 back to test scope
eecee5f [Doris Xin] Merge branch 'master' into stratified
f4c21f3 [Doris Xin] Reviewer comments
a10e68d [Doris Xin] style fix
a2bf756 [Doris Xin] Merge branch 'master' into stratified
680b677 [Doris Xin] use mapPartitionWithIndex instead
9884a9f [Doris Xin] style fix
bbfb8c9 [Doris Xin] Merge branch 'master' into stratified
ee9d260 [Doris Xin] addressed reviewer comments
6b5b10b [Doris Xin] Merge branch 'master' into stratified
254e03c [Doris Xin] minor fixes and Java API.
4ad516b [Doris Xin] remove unused imports from PairRDDFunctions
bd9dc6e [Doris Xin] unit bug and style violation fixed
1fe1cff [Doris Xin] Changed fractionByKey to a map to enable arg check
944a10c [Doris Xin] [SPARK-2145] Add lower bound on sampling rate
0214a76 [Doris Xin] cleanUp
90d94c0 [Doris Xin] merge master
9e74ab5 [Doris Xin] Separated out most of the logic in sampleByKey
7327611 [Doris Xin] merge master
50581fc [Doris Xin] added a TODO for logging in python
46f6c8c [Doris Xin] fixed the NPE caused by closures being cleaned before being passed into the aggregate function
7e1a481 [Doris Xin] changed the permission on SamplingUtil
1d413ce [Doris Xin] fixed checkstyle issues
9ee94ee [Doris Xin] [SPARK-2082] stratified sampling in PairRDDFunctions that guarantees exact sample size
e3fd6a6 [Doris Xin] Merge branch 'master' into takeSample
7cab53a [Doris Xin] fixed import bug in rdd.py
ffea61a [Doris Xin] SPARK-1939: Refactor takeSample method in RDD
1441977 [Doris Xin] SPARK-1939 Refactor takeSample method in RDD to use ScaSRS

(cherry picked from commit b715aa0c8090cd57158ead2a1b35632cb98a6277)
Signed-off-by: Xiangrui Meng <meng@databricks.com>