HADOOP-16242. ABFS: add bufferpool to AbfsOutputStream

[DadanielZ]

• Added bufferpool to AbfsOutPutStream to improve memory usage.
• Verified performance change, results are attached in HADOOP-16242 jira.

[hadoop-yetus]

💔 -1 overall

Vote	Subsystem	Runtime	Comment
0	reexec	21	Docker mode activated.
		_ Prechecks _
+1	@author	0	The patch does not contain any @author tags.
-1	test4tests	0	The patch doesn't appear to include any new or modified tests. Please justify why no new tests are needed for this patch. Also please list what manual steps were performed to verify this patch.
		_ trunk Compile Tests _
+1	mvninstall	1205	trunk passed
+1	compile	29	trunk passed
+1	checkstyle	19	trunk passed
+1	mvnsite	32	trunk passed
+1	shadedclient	769	branch has no errors when building and testing our client artifacts.
+1	findbugs	42	trunk passed
+1	javadoc	23	trunk passed
		_ Patch Compile Tests _
+1	mvninstall	26	the patch passed
+1	compile	24	the patch passed
+1	javac	24	the patch passed
+1	checkstyle	14	the patch passed
+1	mvnsite	27	the patch passed
+1	whitespace	0	The patch has no whitespace issues.
+1	shadedclient	824	patch has no errors when building and testing our client artifacts.
+1	findbugs	57	the patch passed
+1	javadoc	20	the patch passed
		_ Other Tests _
+1	unit	80	hadoop-azure in the patch passed.
+1	asflicense	27	The patch does not generate ASF License warnings.
		3324

Subsystem	Report/Notes
Docker	Client=17.05.0-ce Server=17.05.0-ce base: https://builds.apache.org/job/hadoop-multibranch/job/PR-776/1/artifact/out/Dockerfile
GITHUB PR	#776
Optional Tests	dupname asflicense compile javac javadoc mvninstall mvnsite unit shadedclient findbugs checkstyle
uname	Linux 735d04278974 4.4.0-143-generic #169~14.04.2-Ubuntu SMP Wed Feb 13 15:00:41 UTC 2019 x86_64 x86_64 x86_64 GNU/Linux
Build tool	maven
Personality	personality/hadoop.sh
git revision	trunk / 3758270
maven	version: Apache Maven 3.3.9
Default Java	1.8.0_191
findbugs	v3.1.0-RC1
Test Results	https://builds.apache.org/job/hadoop-multibranch/job/PR-776/1/testReport/
Max. process+thread count	338 (vs. ulimit of 5500)
modules	C: hadoop-tools/hadoop-azure U: hadoop-tools/hadoop-azure
Console output	https://builds.apache.org/job/hadoop-multibranch/job/PR-776/1/console
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[steveloughran]

+1, committed. Don't expect any tests for this as its hard to test, and perf graph shows (a) benefit and (b) manual testing.