JDK-8074023: Clock.system(ZoneId) could be optimized to always return the same clock for a given zone

[nadeesh tv]

Hi all,

Please review this minor change which  optimized 
Clock.systemDefaultZone() and Clock.system(ZoneId)  to avoid creating 
new instance of Clock.SystemClock every time they are called.

Bug ID :  https://bugs.openjdk.java.net/browse/JDK-8074023

Webrev : http://cr.openjdk.java.net/~rriggs/nadeesh-tv-8074023/

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Thanks and Regards,
Nadeesh TV
Oracle IDC, 6th Floor Divyasree Chambers
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